Guides
What Is an APK File? How It Works
An APK is a signed ZIP archive. Knowing what is inside it explains almost every install error you will ever see.
What is inside
| Component | Purpose |
|---|---|
| AndroidManifest.xml | Declares package name, version, permissions and components |
| classes.dex | Compiled application bytecode for the ART runtime |
| res/ and resources.arsc | Layouts, images, strings and other resources |
| lib/ | Native libraries, one directory per CPU architecture |
| META-INF/ | Signature files proving which key signed the archive |
Why the signature is the important part
Because an APK is an ordinary archive, anyone can unpack, modify and repack one. What they cannot do is reproduce the original signature. Android enforces that an update carries the same signing key as the installed app, which is the entire basis of app identity outside a store.
Split APKs and bundles
Modern Play distribution often ships an app bundle split into a base APK plus configuration APKs for screen density, language and CPU architecture. Installing only one part produces an invalid-package error — a common cause of confusion when files are shared outside the store.
Worth knowing: You can rename an APK to .zip and open it — the format is a standard archive, which is exactly why repackaging is easy and signatures matter.