First thing you need to do is get the app "shortcuts". if you dont have the app, you can get it here! Dont worry, its from apple so its not a virus.
Second, you need to get the shortcut that allows you to download youtube videos straight to your camera roll for free. You can get it here!
Next, accept "get shortcut" then accept again. Now you are done with the hard part.
Finally, go to any youtube video and click "share". it will bring you up to different sharing platforms. Scroll to the right and click "more". Then, there will be a "shortcuts" share button. Click it and it will bring up the shortcuts you have. click the shortcut "Download Youtube".
Boom! wasnt that easy. it should be in your camera roll after u ran the shortcut. remember, its free, fast, and easy to do.