Apps You Use Daily —
Decoded
Every app has a tech stack. We dug through engineering blogs, public talks, and GitHub repos to bring you the real story behind 30 platforms — from Instagram to Zomato to MasjidGo.
Social Media
📸
Instagram
2 billion+ users
Started as a Python and Django app. Today still primarily Python on the backend, with React for the web and React Native for parts of the mobile app. They run one of the largest Django deployments in the world.
Tech Stack
💡 Instagram serves over 100 million photos per day from a Python backend.
👥
Facebook
3 billion+ users
Facebook started as a PHP app and still runs a custom version called Hack for most user-facing code. They invented React and use it everywhere. C++ runs the high-performance backend services.
Tech Stack
💡 Facebook serves 3 billion people from PHP code (compiled to C++ via HHVM).
🐦
X (Twitter)
500 million+ users
Twitter originally ran on Ruby on Rails but famously could not handle the load. They rewrote critical services in Scala and Java. The web frontend uses React. Data and analytics use Python.
Tech Stack
💡 Twitter's switch from Ruby to Scala in 2010 was one of the most famous tech rewrites.
Messaging
💬
WhatsApp
2.5 billion+ users
WhatsApp's entire backend runs on Erlang — the language built by Ericsson for telephone exchanges. This is why WhatsApp can handle billions of messages with very few engineers and almost no downtime.
Tech Stack
💡 WhatsApp ran 450 million users with just 32 engineers, thanks to Erlang.
Video Streaming
🎬
YouTube
2.7 billion+ users
YouTube's frontend was originally Python (Django) and remains heavily Python today. Google rewrote performance-critical parts in Go and C++ to handle the scale. The video player and processing pipeline run on C++.
Tech Stack
💡 YouTube processes over 500 hours of video every minute — needs C++ performance.
🎭
Netflix
270 million+ users
Netflix's backend is mostly Java microservices running on AWS. The website and many internal tools use Node.js. Data science and machine learning (which powers recommendations) is Python.
Tech Stack
💡 Netflix runs over 700 microservices, mostly in Java, to deliver streams worldwide.
Music Streaming
🎵
Spotify
600 million+ users
Spotify uses Python for backend services and machine learning (the famous Discover Weekly is Python-powered). Mobile apps are native Swift and Kotlin. Audio playback engine is C++.
Tech Stack
💡 Spotify's recommendation engine processes 4 billion data points per day in Python.
Ride-hailing
🚗
Uber
150 million+ users
Uber rebuilt their core dispatch system in Go for performance. Java handles the main marketplace. Python is used heavily for data and ML. The driver and rider apps are native Swift and Kotlin.
Tech Stack
💡 Uber's real-time dispatch matches riders to drivers using Go services.
Travel
🏠
Airbnb
150 million+ users
Airbnb is one of the largest Ruby on Rails deployments still running. They have moved some services to Java for performance. The web frontend is React. Mobile apps are native Swift and Kotlin.
Tech Stack
💡 Airbnb has run Ruby on Rails since 2008 and continues to invest in it.
Developer Tools
🐙
GitHub
100 million+ users
GitHub is the largest Ruby on Rails application in the world. They use Go for performance-critical services and React for the frontend. The Git internals (LibGit2) are C++.
Tech Stack
💡 GitHub serves 100M developers from a Ruby on Rails monolith.
💻
VS Code
15 million+ users
VS Code is the most popular code editor in the world. It is built almost entirely in TypeScript using the Electron framework. Microsoft created TypeScript partly so they could build VS Code.
Tech Stack
💡 VS Code proves TypeScript can build huge production-quality desktop apps.
CMS
📝
WordPress
43% of all websites users
WordPress is pure PHP and MySQL — the original LAMP stack. The new editor (Gutenberg) uses React. Themes use HTML, CSS, and JavaScript. Plugin ecosystem is the largest in the world.
Tech Stack
💡 WordPress powers 43% of all websites — including some you would never guess.
Reference
📚
Wikipedia
1.5 billion+ users
Wikipedia runs on MediaWiki, which is PHP. The frontend uses vanilla JavaScript and small amounts of Vue. Data engineering and bots are Python. They proudly run on open source from top to bottom.
Tech Stack
💡 Wikipedia runs entirely on PHP and serves the world's 5th most popular site.
Communications
🎮
Discord
200 million+ users
Discord pioneered the use of Elixir for real-time messaging at massive scale. They later rewrote certain hot paths in Rust for additional performance. The desktop app is Electron with React.
Tech Stack
💡 Discord runs on Elixir for real-time messaging and Rust for performance-critical parts.
Auto / EV
🚙
Tesla
5 million+ cars users
Tesla's in-car software runs Linux underneath, with C and C++ for low-level systems. Autopilot uses Python and C++ for AI inference. The Tesla mobile app is React Native (with native Swift/Kotlin parts).
Tech Stack
💡 Tesla's self-driving AI stack runs on Python (training) and C++ (inference).
Payments
💳
Google Pay
150 million+ in India users
Google Pay India was rebuilt using Flutter (Dart) for both Android and iOS — making it one of the largest production Flutter apps in the world. The backend runs on Java and Go on Google Cloud.
Tech Stack
💡 Google Pay India runs on Flutter — proving Flutter can scale to 150M users.
💰
Paytm
350 million+ users
Paytm runs on a Java backend with Spring Boot, optimized for the massive transaction volume of Indian payments. The web is React. Mobile apps are native Kotlin and Swift.
Tech Stack
💡 Paytm processes billions of UPI transactions yearly on a Java-based backend.
E-commerce
🛒
Flipkart
450 million+ users
Flipkart's backend is primarily Java microservices on a custom platform. The web frontend uses React with Node.js for server-side rendering. Mobile apps are native Kotlin and Swift.
Tech Stack
💡 Flipkart processes millions of orders during Big Billion Days using Java microservices.
🏪
Shopify
4 million+ stores users
Shopify is one of the largest Ruby on Rails monoliths in the world, handling millions of merchants. They have invested heavily in Ruby performance. The frontend tools and admin use React with TypeScript.
Tech Stack
💡 Shopify processes Black Friday sales worth billions on Ruby on Rails.
Food Delivery
🍽️
Zomato
80 million+ users
Zomato originally built on Ruby on Rails and migrated some services to Go for the food delivery operations. Mobile apps are native Kotlin (Android) and Swift (iOS) for performance.
Tech Stack
💡 Zomato serves 80M users from a Ruby on Rails core with Go microservices.
🛵
Swiggy
60 million+ users
Swiggy uses Java and Spring Boot for most backend services. Real-time order tracking and matching uses Go. ML for delivery time predictions and restaurant recommendations is Python.
Tech Stack
💡 Swiggy's delivery time predictions use Python machine learning models.
Finance / Trading
📈
Zerodha
13 million+ users
Zerodha rewrote much of their tech in Go for the speed advantage in trading. They use Python for data analysis and reporting. The Kite trading platform frontend is React.
Tech Stack
💡 Zerodha's order matching engine uses Go for low-latency trade execution.
Community App
🕌
MasjidGo
Growing users
A Sadaqah Jariyah project for the Muslim community by OneCity Technologies, built entirely in Flutter (Dart) with Supabase backend. Available on Android and iOS, supports 12 languages including English, Arabic, Urdu, Tamil, Bengali.
Tech Stack
💡 MasjidGo offers Qibla, prayer times, and azan notifications across 12 languages.
Education
🎓
CollegesInfo.org
1,432 colleges listed users
OneCity's flagship Karnataka education portal — listing 1,432 colleges, 182 articles, KCET and NEET predictors. Built in PHP with Laravel framework, MySQL database, and a clean Bootstrap-based responsive UI.
Tech Stack
💡 CollegesInfo lists every college in Karnataka with admission predictors.
DevOps
🐳
Docker
13 million+ developers users
Docker, the standard for application containers, is written entirely in Go. This is one of the reasons Go became popular — Docker showed the world a polished, fast tool that could not have been built easily in any other language.
Tech Stack
💡 Docker is written 100% in Go and runs everywhere Linux runs.
Design Tools
🎨
Figma
4 million+ users
Figma's rendering engine is Rust and C++ compiled to WebAssembly, which is why it runs so smoothly in your browser. The UI is React with TypeScript. They pioneered using Rust for browser-based design tools.
Tech Stack
💡 Figma runs as fast as native software because of Rust + WebAssembly.
Gaming Platform
🎮
Roblox
70 million daily users
Roblox uses Lua as its scripting language for all user-created games. The engine itself is C++. This is what allows millions of teenagers worldwide to build games without learning a hard language.
Tech Stack
💡 Roblox creators have earned over $1 billion writing Lua scripts.
AI / ML
🤖
ChatGPT / OpenAI
200 million weekly users
OpenAI's entire AI training stack is Python with PyTorch. The performance-critical inference parts use C++ and CUDA (NVIDIA's GPU language). The website and ChatGPT app frontend use TypeScript and React.
Tech Stack
💡 ChatGPT was trained using Python on tens of thousands of NVIDIA GPUs.
Internet Infrastructure
☁️
Cloudflare
Powers ~20% of the web users
Cloudflare protects huge portions of the internet using a network written increasingly in Rust. They were one of the first major companies to make Rust a production standard. Workers serverless platform runs JavaScript and TypeScript at the edge.
Tech Stack
💡 Cloudflare's edge network is rewriting Nginx in Rust for safety.
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