The isekai tag is technically applicable and all, I suppose, but this does sound quite different than what you come to expect from that tag. I think this sounds more like a D&D character who adventured in one of the specific settings like Forgotten Realms or something suddenly getting Planescape'd by their DM. Congratulations, you've just discovered that your entire world was just part of a bigger whole. But don't worry, pretty much everything in all these other places still wants to kill you. It's just like home, really.