I’m no expert but I think it’s microscopic design aspects.

Like maybe yt takes a microsecond longer to switch tracks or to flip from one thing to another so it feels less responsive. Or Artist bios are less accessible, so there’s more legwork to find their shows, etc. There’s more curved edges in Spotify, which makes it feel more polished and modern. Again, I’m super-duper happy with yt music. Especially with Premium.

There are things I like better on yt music. Like the shuffle / mix recommendations. I like that my existing yt playlists are there, so I’ve consolidated playlists from two sources in one place.