Help with multiple levels/instances

Hello, I'm fairly new here and have still much to learn, so sorry if I mix up some definitions. Also, my English is not perfect unfortunately, so feel free to ask for clarification please if I'm not clear.

Right now, I'm trying to figure out if it's possible to create a Core game with multiple levels/instances. It would be a first person RPG, as open world as possible, but due to terrain size limit I think I'll need to be able to create more different levels/instances I think.

For example: can I have a desert level (max size) and then at all corners some exits? That once you trigger it, or walk through it, it loads up a corresponding level/instance? For when progressing the game to a different biome or when entering a dungeon, for example.

I found the Portal but if I'm not mistaken, that only allows me to go to a complete different game. I got that working but the loading time (finding server) can be high when transitioning. And also I read that it's not possible to transfer gear/inventory with character to another game, making it not really an option to use for different levels/instances I guess.

Is someone able to tell me if what I want is even possible? Or steer me in the right direction so I can figure it out myself please? Many thanks.