AOSP Folder Structure

art

It contains all the modules related to Dalvik VM, JDK VM

bionic

It is custom C library, other libraries related to Dynamic Linkers

bootable

It contains bootloader, ota and modules of recovery mechanism

build

It contains files for build like envsetup.sh, soong, make modules

compatibility

Compatibility Definition Document (CDD) has all the files related to compatibility.

cts

Android Compatiblity Test Suite, this contains all the modules required to run tests of compatibility

dalvik

this contains modules supported for dalvik virtual machine