I am going on a long a boring flight and want to watch some YouTube videos. But my iPadPro is running out of storage. Is there a way I can put these YouTube videos on an external drive with an SD card and access them?
Sure, as long as you’re able to get the files on the drive and have a way to connect the drive to the iPad. Then you should be able to access the videos in the Files app and watch them natively or you could use something like VLC as well.
I think OP question is if he/she download video from YouTube (because of premium subscriber), can he/she download to specific folder such as USB? I would guess, no, because the downloaded files are buried in system folders inside iPadOS files system which are opened very limited by Apple.
I agree, it's very possible to download the videos to an external drive. As far as I know, you can't do it through the YouTube app (or even from an iPad directly). The Files app on your iPad can access a USB Type C drive/sd card and you can play the files. It's not elegant, but it works.