Hello everyone
What I know about technical things can be written on the back of a stamp !
However, I used to be able to download art lessons to my computer from You Tube by clicking on the revolving coloured balls that turned while said lesson was playing. Since some time now these are no longer available and it’s been frustrating BUT I have found this site and even a rookie like me has been able to telecharge it, and it works 
So if you too (pun intented) are having problems then you might like to try this …It’s available in several languages.
https://fr.savefrom.net/1-comment-télécharger-une-vidéo-youtube.html
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If you particularly want something from YouTube you could just add it to a playlist which would always be available. Obviously you would need to have a YT account but you wouldn’t be taking up any valuable disk space.
You can try this it works though not sure if legal ?
Notube is good, free, very easy to use.
Given that providing the IT infrastructure to do the conversion isn’t free I wonder what is in it for them?
They have a popup ad page you just close.