Mods have to be very innocuous to avoid disabling multiplayer, cosmetic changes only like overwriting flag images, overwriting sound files, changing strings, things like that. I would presume anything related to techtree modding will definitely disable it, especially adding brand new techs, as I’ve had trouble with just a politicianmods file that tried to swap nothing but the image file name used by a revolt.
If this is something for you and your friends then you should be able to play together as long as you all install the same mod and have it turned on.
Additive mods make it so your mod doesn’t need to be rebuilt every time an update comes out and so it can function with other mods. I’d imagine almost all additive modding files with exception to stringmods and sound files will disable multiplayer since the majority modify major game files and it wouldn’t be fair if you could just throw in a techtreemods to give Aztecs 10000% damage and mosey on into multiplayer.
Unfortunately it means if you’re trying to give Musketeers to the Dutch or something like that you either have to sacrifice multiplayer or sacrifice the unique Dutch units/cards/techs by trying to make an ersatz version by modifying the flags, sounds, and strings of another civ to morph them into a facsimile of the Dutch.