How are mlo's made?

I see all these Y-maps, and they look awesome. I was wondering how they’re made. I know how to use Spooner, but I can’t figure out how they made the interiors. I asked some people, and they said that all of it is already in the game. I don’t know what they mean by that. I checked Spooner’s menu, and I don’t see almost any walls, roofs, or floors. Hell, I don’t even know how they deal with the clipping issues that are inside the buildings.

Research how MLOs work in GTA5, it would be way easier to understand as there’s more documentation. It’s pretty similar to what we have in Red Dead Redemption 2.

For free look into Blender and get Sollumz addon and Codewalker for retrieving data in game such as building names to export in CW RPF Explorer etc.

It’d be far to lengthy to explain how an MLO is made but its essentially just a portal when you walk through a door inside (sometimes multiple portals inside one building for each room) there are lots of tutorials on the manner as well as Discord servers for Blender Sollumz and Codewalker with many people willing to point you in the right direction as long as you’re willing to put in the work and learn.