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Google originally did not want to make any money from Android's app store
(www.androidauthority.com)
This is a most excellent place for technology news and articles.
Is it not now? There are lots of actively-maintained apps in F-Droid, and as I understand it anything in F-Droid must at least be buildable with open source tools.
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. https://gitlab.com/android-rebuilds/auto Debian 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.
I'm not sure, but F-Droid does state:
They've been pretty consistent about their requirement that everything be open source.