What characters can walk/exist outside the universes itself?

The outside of the universe in Bleach is called Garganta. The universes and dimensions exist inside of it.

Yhwach can open a portal there on his own. (It's not the Dangai because that has a floor unlike Garganta)

The Garganta is a "void" that covers the "dimensions/universes" as if they were "planets" and garganta is their "outer space".

Dangai is a physical tunnel with its own time and space.