Question How to de-render everything outside of a room while in one?

I apologise if the title is a bit confusing, I'll elaborate about what I mean here.

I am currently developing an isometric game with Unity, and I want my entire game to be sectioned off into "rooms".
I would like to know how to make it so that when you are inside one "room", the rest of the game de-renders or disappears.(Think like Pokémon, but isometric view, and only 2 walls and the floor are visible along with the objects in the room).
The idea is to have a brief fade-in fade-out when entering another room to allow the objects and visible walls and floors to render in.

I don't know much about C#, I'm a beginner programmer and this game is for a college project. Could anyone help me with how I could pull this effect off, or something similar?
Thank you

 

Broadly this is called occlusion culling.

https://docs.unity3d.com/Manual/OcclusionCulling.html

Otherwise you can do it yourself with trigger volumes and some code, if you insist.

Remember also this is likely a completely unnecessary optimization until you get to a fairly large level size. Unity is an absolute BEAST of a game engine.

 

The easiest way I could think of would be to partition content into interiors (the rooms) and exteriors (everything but the rooms). You just parent everything that belongs to a room to one such object, then you can toggle its visibility however you need it.