Remixing!
how do unknown artists remix songs, how do they get their hands on the stems?
https://soundcloud.com/swagesound/major-lazer-dj-snake-lean-on-bassex-x-swage-remix
I’ve never heard of either of these artists, how did they get the stems..help me
its simple… we steal them 
we use software and hardware to extract vocals from tracks… we use a combination of software tools (ableton) music theory and our ears to get the melodies and progressions to remix.
for instance, this was a bootleg done both using abletons midi detect functions and by ear
https://soundcloud.com/calgar_c/solarstone-shield-pt-i-calgar-c-remix-free-download
once you get your stems created then you go to town and remix