Like gta online interior, placed under map. But I'm not sure if i can create new enterable interior. I'd like to create enterable building when it's not supposed to. Is it possible?

I don't know what it's called but that's the thing i wanted to ask you guys tho. It seem that gta v ground looks flat, and you can see watch dogs 2's it feels there is some height between soil and the I don't know, grass or moss? So what makes flat texture feels like there's some height but without adding polygons beside the bump map? Even roads feels flat.