I'm surprised preservetube didn't get a mention anywhere in the op of the thread.
I find it to be a pretty useful tool to archive youtube videos and it's simple to use too, just copy and paste the youtube url of the video you wanna save. For a quick example, I saved these 2 videos below directly on preservetube.
https://preservetube.com/watch?v=8JBoPG_TltEhttps://preservetube.com/watch?v=LFKl-GPFJKoJust a fair warning, don't go into the latest tab, it's not exactly full of family-friendly content over there.