A lot of them are on labels which give them unused acapellas. Or they can message singers and work with them because they have a following. What has worked for me is finding a song you like (that doesn't already have a million isolated vocals videos on YouTube) and downloading it (just google YouTube to mp3 converter) then, go to https://www.lalal.ai . It's a website which you put the mp3 in and it isolates the vocals/drums for you. It should be free, but recently they've been getting weird about a membership or something. I hope this helped dog