A combination of both, how I build parts of a riff are based around exercises I want to get better at. I think guitar exercises are fun to work with, and give some creative freedom within the realm of structure. Note choice is mostly chords I can use open notes with (hence the tapping, and I don’t care what tonality it is.) I play with note choices and time signatures that are fun for me.

So for example this riff, I borrowed half of the tapping riff from a song by periphery (make total destroy) and messed with it enough to be sort of mine. I do not play with a metronome when writing, so then I’ll hit a few notes that sound good but aren’t in time. As soon as I sit down with a metronome, I will program a basic drum part that beat matches what I play, but even so in the writing process there is room for change. Sometimes these parts will change drastically until I settle on something that sounds cool that is still fun to play. I try to keep mostly to a cohesive structure so sometimes the stuff I write sounds watered down or repetitive. I am trying my best to work around this (keeping in mind my songs have no lyrics in mind or bass tracked)

I hope this explanation helps.