It's a collection of demiplanes
Specifically, the Domains of Dread or Demiplane of Dread are demiplanes controlled by the Dark Powers about whom bugger all is known, save that they attract evil or despairing creatures like Strahd von Zarovich, who is imprisoned in the Demiplane of Dread known as Barovia.
Are Domains of Dread separate from the Prime Material Plane?
Yes.
How does one get there from the aforementioned plane?
However the DM decides.
A wardrobe in a creaky mansion. Tieing at 'Rock, Paper, Scissors' 100 time in a row. Through the looking glass. Over the Rainbow. By it materialising on this plane and taking you back when it goes. By getting hit on the head.
If you try really, really hard it might be possible to come up with a way no one else has thought of.
Is it even possible?
Yes.
If not, is it possible to "summon" the creature who owns the domain into your plane, thus bringing their domain with them?
Not applicable.
