Some interesting info

If map has content for example for rail and street module, but I only own street module, how will that work?
You will see all content on the map, but you can't use the content yourself, which you don't own. For example you'll see AI trams driving, but you can't drive them yourself.

Source https://www.lotus-simulator.de/forum/index.php?thread/474-how-will-the-different-modules-work-rail-module-street-module-etc/

Can I join a multiplayer game, if a hoster has set up DLC vechiles and/or DLC map?

DLC vehicles - yes, same logic applies as I've written above regarding owning modules.
DLC maps - yes, if the multiplayer game/session has more than 1 maps enabled. You just can't join the map that you don't own yourself.

Source https://www.lotus-simulator.de/forum/index.php?thread/4076-some-multiplayer-suggestions-dlc-compatibilty-server-joining/

If multiplayer session hoster leaves / crashes from the game, what happens with the session?

Currently the session will stay alive and other players can continue playing. When hoster restarts their game, they get promted to return to the session.