Well, I don’t think SB is necessarily a different dimension. It’s almost like a world within our world. Where time was frozen. SB is a part of our world, but maybe it wasn’t before the curse. For example, if the EQ created SB when she enacted the curse, then it would be a random magic town in the middle of Maine.
But maybe the characters were transported to SB from the EF.
Either way, none of the characters in SB could leave SB besides Emma, August, and Henry because they were all cursed, except for those three.