Both Youtube and Spotify allow you to listen for free with ads.
But to humor you. Youtube is for video content primarily. Youtube Music is a premium service that costs as much as Spotify does and has a whole lot less content.
As someone who listens to a lot of music. Spotify offers me a way better UI/UX for consuming music than Youtube Music does.
Yes but this sort of makes me question, if it is on Youtube for free, why are we paying for it on Spotify and Apple Music?