In either case, if you download the mods from Bethesda’s official site, you should read the procedure as stated in their official support portal. In case you have a console (Xbox One or PS4), you simply need to add the mods from Bethesda’s library. For instance, if you download a Skyrim mod from Nexus mods, you need to follow their official guide to install the Skyrim mods manually.
It usually differs depending on the portal you download it from (for PC). There isn’t any single solution to install a Skyrim mod.
