On the face of it, the most popular open-source operating system is Android, which is based on a modified version of the Linux kernel. The issue is that what many people think of as Android is not ...

The preferred license for the Android Open Source Project is the Apache Software License, Version 2.0 (“Apache 2.0”), and the majority of the Android software is licensed with Apache 2.0.

ACKs are the main branches of the Android Open Source Project (AOSP), which includes Google's own Android implementations for third-party companies working in the Android ecosystem.