Would like guidance on custom map support


I’m making a game which will consists of a variety of models, forming a level/map, and some models will be able to move such as doors or vehicles. Each map has objectives/tasks to complete in order to win.
I would like to allow users to be able to host their own servers, which allow players to play on custom maps. So as I’ve never done this before, I would like to ask:

> How would I go about loading the map models and the map’s objectives into a game? I mean, for example I could connect to someone’s server and complete the objectives which they’ve programmed into their map.
This is like on Gmod, where users join and download all of the level content. I just don’t know how to get them to play with the downloaded content.

Any guidance would be very useful