Not one of the devs so I don't know about how Joplin handles files that are too big
However, it does seem like you can just store them locally (as in, in your files and folders but not in a note) and create a link that will open the file by holding alt and dragging/dropping the file into the note, that way no matter how big the file is, all the note needs to contain is a link to it in text, hope that helps!
(Do note, while you've mentioned syncing is just for having a backup for you and this will work fine for that, it might break if you do sync a second device and Joplin can't find a file in the same place, just an fyi for possible future concerns)
As for "basic computer knowledge", imo don't feel bad or confused if any of the forum posts go over your head a little bit, lots of people here are technical users so we use a lot of jargon, but I guarantee people are happy to help and explain stuff if you ask them! 