The Android kernel is build over linux kernel but been heavily modified. They done changes to Threading, power management and security.
The runtime application executed compiled code on the device, the original runtime was Davlik Executables (DEX) that was using Just-In-Time compilation. They used Davlik until version 5.0 which then they moved to Android TuneTime (ART) which was 20 time faster. This was up to Android 7.0 which then they moved to 'Profile Guided Compilation' that uses both JIT and ART.
The Java API Framework is used by developers to communicate with the underlining operation system using API's. Enables developers to communicate with API's such as ActivityManager, PackageManager, NotificationManager, TeleponyManager and more.
All Apps run as a process that runs under specific user (new user for each app). all files that created by the app will be using the same user.
That way each app has privilege separation and own permission module. that way one application won't be to access other application data.