I would like to know the same thing but mostly because some mods from a friends server are taking up like 25% of my space

引用自 Crack Mouse：

I was wondering where the mods are downloaded to so i can copy them to my addon folder or something and start my own server with the mods.

You don't need to copy to a addons folder to start a server.

If you are looking to create a P2P server just install it as a addon like any other and start your server.
If you are looking to create a dedicated server with SRCDS, follow this tutorial to use a workshop collection and add this addon to your collection: https://wiki.facepunch.com/gmod/Workshop_for_Dedicated_Servers

引用自 TitaniumX1001：

I would like to know the same thing but mostly because some mods from a friends server are taking up like 25% of my space

I don't recommend doing this because addons are installed in some layers and deleting the wrong stuff can corrupt your game.
But if you are decided to do it anyway, you can find a guide easily: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=343468221