I think this post from masagrator should answer all of your questions.Bit of a slightly unrelated question:
How did you make this?
I know that unlike the steam version you can't just YEET a bunch of custom PAK files in the right directory and have it work straight away.
So obviously that would mean that the only other option is to unpack the base game's own PAK file, unpack the PAK files of all the mods you want to install on switch, merge all the folders together and overwrite where necessary and then repack that back into the base game's PAK file.
But how does one unpack/repack Unreal Engine's PAK files?
I only know how to export models from PAK files, not repackaging
https://gbatemp.net/threads/how-to-unpack-and-repack-unreal-engine-4-files.531784/