Traditional mods are loaded as pak files in the same directory as the main game pak (these pak files are created using a tool such as unrealpak)
The content inside these paks files will just be content that has been created and then cooked using the same version of unreal as the game in question
These mods will likely need a _P at the end of their name to remove the mount point of the pak and clarify it as a patch so that it overwrites original game content
The mods should be place in a mod folder (~mods works for every game) to ensure proper load order (so the main game pak doesn't overwrite the mod's content)
Paks are loaded alphabetically in each directory
Mods can also be loaded via modloader frameworks such as the Unreal Mod Loader (a tool that allows the execution of any arbitrary blueprint code) and UE4SS (a tool that allows Lua scripting via reflection)