Android SDK source code is available, in theory and in theory you can build yourself. In practice binaries provided by Google come with restricting licence how you can use them while source is so scattered around weird control systems that noone knows if it’s actually complete source and possible to use.
There was a project to provide FOSS builds of the SDK, but is unmaintained. gitlab.com/android-rebuilds/autoDebian also has android-sdk in it’s repos, but 23 is the max API level now.
I don’t know how F-Droid build apps today, it seems like a big problem.