Designing Mobile Applications Like U.S. Big Tech: iOS and Android System Design

A practical mobile system design course on structurally designing iOS and Android apps

Mobile apps aren’t complete just because you’ve built polished screens.
True mobile system design begins when the network goes down, the app is terminated, data is duplicated, and countless events arrive simultaneously.

This course is a hands-on journey where you design Instagram feeds, WhatsApp messaging, Uber location tracking, Netflix offline downloads, Spotify streaming, and ChatGPT- and Claude-style AI apps, building the mobile architecture mindset expected at major U.S. tech companies.

We do not memorize a completed architecture from the start.
Starting with the simplest implementation, you will directly discover and solve problems such as data inconsistencies, slow screens, duplicate messages, battery drain, app termination, and deployment failures.
Through this process, you will gradually master the principles of offline-first design, Sync Engines, state machines, caching, retries, real-time connections, Push Notifications, observability, and AI-native architecture.

By comparing the platform constraints of iOS and Android, you will learn how to design mobile systems that are fast, stable, and capable of recovering from failures, going beyond feature implementation.

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After completing the course,

• You can systematically design large-scale mobile services from requirements through failure handling.

• You can design an offline-first architecture that can recover even after network disconnections and app termination.

• You can design real-time systems that include message ordering, reconnection, read receipts, and push notifications.

• You can make iOS and Android architecture decisions that account for app lifecycle, battery, memory, and network constraints.

• You can design an AI-native mobile app architecture that combines LLM streaming UIs with on-device and server-side AI.

• In interviews or real-world work, you can logically explain the reasons behind your design decisions and the trade-offs, rather than simply listing technologies.

Features of this course

1. A course where you experience problems first, solve them, and learn the underlying principles.

This course does not explain a completed architecture from the start.
It begins with the simplest structure, then has you directly confront the problems you will inevitably encounter in a real-world service.

For example, with image loading,
we start by “simply fetching and rendering the image from a URL,”
and then directly observe issues such as duplicate requests while scrolling, incorrect images being displayed, increasing memory usage, and cache invalidation.
Only then do you understand why concepts such as caching, request deduplication, cancellation, decoding, and disk caching are necessary.

In other words, this is not a “course that introduces technologies”
but a course that helps you internalize why problems occur and how to solve them structurally.

2. A course that designs big-tech case studies from a mobile perspective, all the way through

We don’t simply introduce the Instagram feed, WhatsApp messaging, Uber location tracking, Netflix offline downloads, Spotify streaming, or the ChatGPT and Claude mobile apps.

We design each case from start to finish according to the following flow.

• What requirements are there?

• What should the mobile client be responsible for?

• Where is the data stored?

• How does it work when offline?

• What needs to be restored when the app is terminated?

• How are real-time connections and push notifications divided in terms of their roles?

• How should performance, battery usage, security, and observability be designed?

So by the end of the course, you will progress beyond simply thinking, “Oh, I guess this is roughly how such an app would be built,”
to the point where you can actually explain your design proposal.

You’ll learn the following:

Fundamental Principles of Mobile System Design and Data Architecture

In this section, you will learn the core mindset of mobile system design.
You will understand the difference between implementing features and designing systems, and clarify the scope of what a mobile client should actually be responsible for.

In particular, it focuses on the following topics.

• Mobile System Design Approach

• State Management and Data Layer

• Single Source of Truth

• Caching and cache invalidation

• Offline-first

• Sync Engine

• Optimistic updates

• Conflict resolution

• App Lifecycle and Process Death Handling

• iOS, Android Background Execution Models

Through this section,
you will move beyond the level of “an app that displays data received from the server”
and establish the principles for designing a recoverable, long-lasting mobile data architecture.

Real-time systems, performance optimization, Push, AI-native architecture

This section covers the most challenging problems encountered in real-world services.

• WebSocket, SSE, and Polling Comparison

• Message ordering guarantees and read receipts

• Push Notification System Design

• Image Loading and Caching Pipeline

• Performance optimization for large-scale lists and feeds

• Memory, Battery, and Network Budget Design

• Mobile Observability Architecture

• Phased rollout and operations strategy

• On-Device AI vs. Server AI

• LLM Streaming UI

• Agentic Mobile UX

Through this section,
you will not learn the concepts of each technology separately; instead,
you will develop the ability to assess real-time capabilities, performance, operations, and AI holistically within a single app architecture.

The person who created this course

• Silicon Valley Survivor | American Snail 

  Drawing on experience and expertise gained at the forefront of the Global Tech Scene, this course presents a path for non-majors to overcome technical barriers and become the owners of their businesses.

  • Current) Founder of a Silicon Valley AI coding agent startup

    • Operating the self-developed AI tool 'Snailer CLI' (18K+ downloads)

    • Selected for the Google for Startups Program

  • Former engineer at a major U.S. tech company and promising startups

    • Gave up an Amazon final-stage opportunity to start a company

    • Silicon Valley AI FinTech Startup Engineer

    • OpenAI / Meta / Apple / Adobe / Amazon Full-Stack Fellowship

    • Domestic search engine portal, fintech development

    • AI startup AR/B2B/SDK development

  • Proven educational expertise

    • Double major in Computer Science and Business Administration at a four-year university in Seoul, along with extensive startup experience

    • Produced 1,800+ students cumulatively; 4.5K+ followers on Threads and 480+ Substack followers

    • Threads: https://www.threads.com/@rich_l_2024?igshid=NTc4MTIwNjQ2YQ==

    • Substack: https://open.substack.com/pub/1234373801

I have never viewed mobile app development as simply “creating screens.”
In practice, when you build an app, the problems begin after the screens are built.

Images load slowly, networks drop, apps crash, real-time connections become unstable, caches get tangled, and users feel for no reason that “the app is acting strange.”
What you need at that moment is not another new library, but a systems perspective.

I have consistently worked on architecture, performance, real-time processing, streaming, data flows, and server integration issues in real-world service environments, with a focus on iOS development.
Through my experience with search portals, fintech services, vision/AR-based apps, real-time audio, video features, and mobile-server integration architectures, I came to realize one thing in common.

Many developers learn how to “implement features,” but
it is difficult to systematically learn “why architectures break down” and “how to design them.”

This course was created specifically to bridge that gap.

Instead of listing theory,
the course is designed to start with the simplest implementation, reveal problems, and teach the underlying principles as you solve them.
That’s why the goal of this course isn’t to introduce a lot of technologies, but to
help students determine the right design direction on their own when they encounter a new mobile service.

Do you have any questions?

Notes Before Taking the Course