Apps for Android devices are written in various languages:
Kotlin
C++
Java
.apk) are files which an app is installed from and include required content that is required for app execution. .apks are archive files (1+ files along with metadata)..aab) are also archive files, but do not contain content required for execution. AABs are used in distribution of software that may be used by a variety of devices with particular hardware or operation differences, so the eventual APKs may differ from the some AAB.Java apps are comprised of four basic components:
Activities: each “screen” with a respective UI. An app may have multiple, and the starting activity does not have to be a single defined one. Activities are subclasses of Activity.
Services: Subclasses of Service – “background” functionality, including while another app’s activity is focused. Chains of service dependencies (Bound Services) may exist.
Broadcast Receivers: Allow for resources to interact with an app outside of activities to provide it with data or trigger a service. Subclass of BradcastReceiver as individual Intent object.
Content Providers: Subclass of ContentProvider– manages stored data using a URI convention.
Android OS is Linux
Individual apps function as respective users on that system
Apps have segregated file permissions
Each app runs as a virtual machine paralell to (and isolated from) others
As a general (but not unbreakable) rule, each app only has access to the minimum amount of resources required to function
The device user must opt into additional hardware/system permissions