No matter the manufacturer, every Android phone has one thing in common: its software base. Manufacturers can heavily customize the look and feel of the Android OS they ship on their Android devices, but under the hood, the core system functionality is derived from the same open-source foundation: the Android Open Source Project. After over 16 years, Google is making big changes to how it develops the open source version of Android in an effort to streamline its development.
Quite true. Linux and all modules loaded into it are GPL licensed. The userland and tooling on the other hand can be licensed however. They are free to close source on anything except kernel code.