I would start from the docs. There is pretty good reaper manual available from its homepage, covers everything from basic "how does one creates the prohect?" to relatively advanced topics.

Most of the plugins I encountered have no native Linux version, but yabridge deals with most of them quite good. I personaly use it paired with Play on Linux to logically separate different groups of plugins and everything works like a charm.

That sounds great, but I was not able to find how a developer can meaningfully contribute to this project. But Readmeisounds promising.

You might be interested in Whisper Cpp, there is even an "example" android app in the repo. If you are looking for complete solution/app, I would probably look into apps that incorporate this lib.