How Apps Work
A course for API developers. Eleven lessons, read in order; each builds on the ones before it.
You already know how to build and call APIs. This course explains the other half of the picture: how the apps on a phone actually work, from the moment a user taps "Sign in" to offline sync, payments, real-time updates, and mobile security. No mobile experience is assumed.
One idea runs through the whole course: a mobile app is an API client, and the server is the only ground you control. Every lesson arrives at that idea from a different direction; the capstone shows all of them recombined in real apps.
Part I: Foundations
01 Web Apps vs Native Apps The core distinction, the store rules, PWAs and wrappers, and what real businesses actually build. 02 How Apps Actually Work Apps as API clients: the four layers, token auth, the hostile lifecycle, the release pipeline, and monetization.Part II: The core flows
03 A Login Flow End to End "Sign in with Google" request by request: PKCE, the ID token, and the seam where the provider's dance meets your own API's auth. 04 Push Notifications End to End Backend to lock screen via FCM and APNs: device tokens, the message-type grid, token hygiene, and notify-then-fetch.Part III: Building
05 Cross-Platform Frameworks Compared Flutter, React Native, and Kotlin Multiplatform as three bets about what to share, with the decision table. 06 Your Path to a First App The zero-cost Expo path for a Python developer on Windows: project choice, toolchain, five stages, shipping options.Part IV: The hard parts
07 Offline Data and Sync The five-rung ladder, single source of truth, the outbox pattern, conflicts, and the five features of a sync-ready API. 08 Payments in Apps Store billing end to end, the subscription state machine, sell-on-web unlock-in-app, and the one-entitlements-table architecture. 09 Real-Time and WebSockets Polling to SSE to sockets, what a websocket really is, the stateful-backend cost, and when polling is the right answer. 10 Mobile Security Three attackers, two public-by-default rules, keys and pinning and storage and attestation, and the 80/20 checklist.Capstone
11 App Architectures: Eight Case Studies WhatsApp, Uber, Spotify, Instagram, banking, Notion, McDonald's, and a reading app. The eight patterns remixed, ending in the pattern-by-app matrix.Get the free email series
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Build the server half
Every lesson here treats the app as a client of an API. Mastering APIs With Python teaches you to build that API yourself: 30 chapters in Python, from your first request to a deployed, tested service. Three full chapters are free.