I think that's what's called procedural audio/graphics. For audio, it's like building synthesizers into the game, to then be able to play the music/fx as a program (sort of like sheet music) instead of an audio file.
The music doesn't have to be completely synthesized.. Most early computers would simply reuse the audio snippets as "samples", and program the pitch and timing in a "tracker"
Listening to the Flappy Birds music, which I admit that I did, then all of the music could be done in a tracker, even if it might not have been originally.
It's a bit of a lost art form, but there are several kilobytes to save!
Personally, I have been considering making a HTML5 game, including the sonvix DLL, to make it really easy to put tiny yet advanced music files into a cross platform browsable game. Haven't gotten around to it though..
Buddy super mario bros was coded in 6502 assembly and required the devs to use every data saving trick in the book like metasprites, sprite flicker, tilesets, color pallets, etc.
Android apps can be notoriously overhead bloat from Java, so for the modern age this is decently impressive.
Android has a massive built-in library of supporting functions that abstracts away most of the differences between devices, including support libraries for older versions of Android, and Flappy Bird is almost the "hello world" of gamws writing.