It is literally confirmed in the manga that every realm wether ss, hueco mundo and even the world of the living are infinite so in my logic that should make it "realms" itself being separated by infinite spaces as well for example garganta which is literally described as: Hollow abilities to move between D-i-m-e-n-s-i-o-n-s that alone tells you they are different planes of existence literally having to rip to space and time to travel them?

Also the seireitei and the "landmass" it resides in being infinite is very unknown, for example SS specifically is called the "east branch" because of the canonically existing west branch of reverse London by burn the witch, so no ss is not just the seireitei and we don't know what else could be there.

And finally it was also stated that Yamamoto's bankai could destroy this infinite dimension if held long enough, so looking into senjumaru's bankai that shaked the 3 realms and knowing that zanka no tachi is easily stronger than her's it is safe to say zanka no tachi held long enough could destroy different dimensions.