Mods distributed as pak files can be placed in the folder '~/Documents/Larian Studios/Divinity Original Sin 2 Definitive Edition/Mods'. These can be downloaded from the Nexus Mods website or the Steam workshop. In the later case there is a website
to download mods, as well as a
guide to do so with a command line version of the steam client.
Note that this guide mentions the App ID changes after updates, but it does not. While the steamdb link may be useful, just for checking the App ID, the Steam store page or Workshop page URL would be easier.
For mods distributed as files/folders, presumably they would need to be copied into the corresponding folder in the game's app container. I don't recall seeing any feedback about doing so with the Mac version, though.
