Easiest way to put music over YouTube video

I'm doing a presentation which involves putting different songs over a piece of footage. The footage I want is on YouTube. I can download videos from there easy enough, then I assumed I'd add the songs in iMovie. The songs are slightly tricky though. They're all on my Apple Music but obviously copyright prevents me using them. Is there another easy way of putting them over the footage? Would I be able to do it if I bought the tracks?