How does MLOs work

Hey. I am learning to make MLOs and so far so good. Though I would like to learn how do MLOs get displayed to the client.

When does an MLO get displayed for a player? Does it depend on the LOD distance? Does it depend on the portal flags?

Is there a way to make an MLO only get displayed when an user is trying to go trough a door? In this case, when somebody just drives or walks past a building they wouldn’t need to have it displayed.

If an MLO is separated in several parts and connected with portals, does the whole MLO gets displayed to the user or only the part he is in?

I just want to understand these things so I can make more or less performance friendly MLOs. Especially since displaying MLOs(without any transparent windows or doors) to an user when he is not going to actually enter the buildings are pointless.

Thank you.