The Squackleverse is the all-inclusive physical universe that exists. The Squackleverse does not include other Realms, per se, but colloquially they can also be referred to as "part" of the Squackleverse.
The Squackleverse is an "Earth-centered" universe that also allows there to be "other planets" and celestial bodies. No planet is bigger than Earth, however they do exist.
There is only one The Sun, which all celestial bodies gain their warmth from.