Not certain if this answers the question, but it seemed like you're generally expected to install Android Studio to get the correct build versions of all of the tools and libraries. I guess theoretically you could repackage them yourself, but also not entirely clear why you would—other than perhaps download size. The tools can be driven externally, once installed, but so could XCode projects (with `xcodebuild`).
They've always offered a bundle of the command line tools separately to Android Studio:
https://developer.android.com/studio#command-line-tools-only
This is not an expectation, no. Libraries are managed via Gradle or whatever build system you use. Android-specific host tools are Gradle-managed, installed via the sdkmanager tool, or managed via other means; I maintain a repository to install them via Nix [0], and many Linux distributions package them. The Android Studio IDE is not required, and doing so would pretty much break everyone's CI setup.
[0]: https://github.com/tadfisher/android-nixpkgs