There’s Finamp, a music client for Jellyfin with offline playback. I’ve not used it personally yet, but with Spotify ratcheting up prices again I’m in the process of switching to self-hosting my music library. When that’s up and running it’s at the top of my list for Android clients.
You are describing a state of software development that has existed since the introduction of punch cards.
Practically every business I’ve worked at has had some internal library or repository of commonly used behavior that can be included in day to day projects.