What made these fun to make was the fact that some really smart people put a lot of time and effort into making a library that will allow you to play midi, “skin” windows HWND surfaces, co-routines, and a high level abstraction over win32 functions. Man, those were the days. These could be cranked out in a matter of hours for any new software as much of the market used the same few vendors or algorithms very similar.
We didn’t know what we know today and so every turn felt like a discovery.
I think most played Protracker, XM, S3M, IT, etc. modules, not MIDI. They typically used very short samples, a style which was called a 'chiptune', songs that were made sound like they came from some eighties microcomputer.
More recently the definition of 'chiptune' shifted to specifically mean music from 8-bit sound chips.