V.E. Schwab, the shades of magic series is exactly up your alley, sounds like.

there are multiple realms that only a few can cross over - different versions of earth, with only a few connection points that overlap.

at least one realm, Black London, has been locked away because of bad things happening in history. those become clear as the series moves on.

there's another book that is kinda old, but involved some guy traveling through and ascending through multiple layers of reality, and his ultimate confrontation with the magic source.. maybe Imajica by Clive Barker, but I'm not sure that's the one (but very similar premise).