[Question] What are examples of simpler universes that have been described in order to explain a concept from our more complex universe?

Sometimes there’s a concept that can be difficult to understand when entangle with everything else that needs to be understood about our physics.

If you isolate that concept in a simpler universe, it makes it easier to explain how the concept works.

What are such examples?

(I feel like I asked a similar question somewhere at some point, but can’t find it)