I listen to a lot of music. When I hear a riff/melody/progression that reminds me of another song, I see if it works. Usually it doesn’t, so I try something else. I have a moderately large-sized database of multitracks at my disposal, so I have a lot to choose from. I’m keeping my sources close to the vest, but suffice it to say there are some smart people who have reverse engineered some popular rhythm-based video games, and I’ve been fortunate enough to obtain some of the resulting files. Also, most great pop singles of the 80s (like Wham!) have instrumentals on the B side of the 12”, and a lot of modern pop songs have instrumentals/acapellas on the CD single. Or with creative editing, you can make your own instrumental sometimes.
IMO, phase inversion is garbage. You need two identically mastered, lossless, perfectly in-phase versions of the song to get anything usable, and the result usually still sounds like an alien walkie-talkie being dropped down a sewer pipe. I’ve heard maybe 10 good sounding phase-inverted acapellas out of hundreds downloaded. Which is a shame, I’ve got so many potential mates for Cee-Lo Green’s “Fuck You” but every DIY acapella has artifacts of the background vocals which clash keys with almost everything.