unit – isometric occlusion sacrifice

I am creating a 3D isometric game in Unity using a standard 3D environment with a fixed camera at 35 degrees from the horizontal. While the player is outside the buildings, the entire building is visible (which is a completely 3D construction model) and when the player enters the building, everything on the floor on which the player is located will not be visible. , the walls closest to the camera will also not be visible. In addition, when the player is outside the building, the interiors of the buildings must be occluded.

I have spent the last few days reading about the frustrum sacrifice and the occlusion sacrifice, but there really isn't much information about this beyond the basics of sacrificing things behind the camera. Information or tutorials on selection areas and selection portals seem to be even scarcer.

Does anyone have any advice or advice on how I can achieve this, please?

Here is an image I found on the Internet that shows an example of what I am trying to achieve.
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