01. According to my firends spotify sound quality is higher. It can be because they offer 320kbps while yt music offer 256kbps
02. Can change. In my mobile, when i tap it it automatically opens mobile default eq settings
03. They are is a share button and can share the album as a link. But i haven't tried it
I mainly use yt music because I listen mostly old songs and covers. In yt music you can listen any youtube listed audio/video song as a music. Another is; since yt music is free when you buy youtube premium it is very cost effective rather than spending a separate portion for another music app.