2d room in 3d level?

I have an idea for my game that I don’t know how to execute, I have a house and I want each room to be a small part of a different type of game. Ex. One room could be a 2d platformer, another could be an 8-bit top-down arena, or a 3d puzzle. How could I do this?
If I need to transfer to another map, how do I switch my character into another level seamlessly? Should I make a loading screen? And if so, how do I do that?

Sorry if these are too many questions.