How Android Really Works
What happens when you tap an icon, what an Activity is, what the manifest declares, and a plain-English answer to the question everyone asks: what is Gradle and why is the first build so slow?
What happens when you tap an app icon
Trace the half second between your finger and the first frame — the launcher, the message it sends, the system service that answers it, the pre-warmed process your app is torn from, and the first line of your own code.
2.2Activities and the lifecycle
An Activity is one screen and the door Android knocks on. Learn the six moments Android announces, why rotating the phone destroys and rebuilds your screen, and why a counter resets when it does.
2.3The AndroidManifest — your app's ID card
One small XML file is the only part of your app Android reads before running anything. Learn what every line of it declares, why minSdk is no longer in it, and what the manifest merger does with the libraries you use.
2.4Resources: strings, colours, drawables
Everything in your app that is not code lives in the res folder. Learn why text belongs there rather than in your Kotlin, how folder name suffixes let Android swap resources for dark mode or another language, and what the R class actually is.
2.5What Gradle actually is
You press Run, something takes over for a while, and an APK appears. That something is Gradle. Here is exactly what it reads, what it fetches, what it compiles and what it packs — in plain English, with no hand-waving.
2.6Why the first build takes so long
Your first build took minutes and your second took seconds, and nothing was broken. Here is exactly where those minutes went, why later builds skip almost all of it, and why a small dependency list matters far more on a phone than on a laptop.
2.7minSdk, targetSdk, compileSdk
Three numbers, sitting three lines apart in the same file, meaning three completely different things. Learn which one decides what you may type, which one decides who can install your app, and which one is a promise about behaviour.
2.8What an APK really contains
An APK is a ZIP file with a fixed set of contents. Open a real one, entry by entry — the dex files that hold your code, the compiled resource table, the binary manifest, the native libraries, and the signature that proves it came from you.