No, we can set a specific size on each usage of the file (meaning: variable custom sizes of the same file, depending on our wishes).
Those thumbnails will be generated by default though, yes.
Average behavior for auto-generated uploads, intended for YouTube Music. This is just an automated release process, so it wouldn't appear on the band channel anyways but the releases tab (if it was an own release by the band though, not samplers).
Besides High Rise being an EP and Big Empty (Aborted) not a title, but just a note of the song Big Empty not getting fully performed = aborted, that should be correct.