Audio-to-MIDI
Listening to a recording and writing out which notes were played, when and how hard, as MIDI notes.
For a single melodic line it is essentially pitch detection plus a rule for where each note starts and stops, and it has worked passably since the guitar synths of the 1980s. Polyphonic material is a different animal: when a chord rings out the harmonics overlap, and one fundamental can look like an overtone of another.
Celemony's Melodyne introduced polyphonic analysis in 2009, and Ableton Live 9 added commands in 2013 to turn melody, harmony and drums into MIDI. Spotify's open-source Basic Pitch from 2022 uses a neural network. The output still needs cleaning up in a piano roll; ghost notes an octave up are the genre's signature.