Thank you, everybody. I won't address each of the last 6 posts, since they're all pretty much on point. I can just say that I'm not really keen on upgrading my old Mac, just so that my new iPad will play nice with it. It's just not worth the hassles, merely to be able to save videos on the iPad.
This is the thing; over the years I've used IDM to download heaps of instructional videos from Youtube, and saved then on an external drive. When I want to watch one, like for instance for my film-making hobby, I just plug in the drive and watch the videos on another comp at my leisure, with no internet connection. That is what I had been hoping to be able to do, with the iPad.
But with all the fiddling around and the downloading and upgrading etc etc ad infinitum, it just seems like too much trouble, (and risk, tbh). Every interaction on the net is fraught with risk, and I can do without that. I did think about upgrading the Mac to Big Sur, which I read is the only available uograde for a 10.9.5 machine, but I couldn't find a download anywhere. But also I thought of the risks of my existing software being unusable if I did upgrade. That's always a possibility, I read.
What I'll do is just use the iPad to watch the Youtube vids online, without downloading them.
Thanks again, everyone, for your generous and helpful input. It's much appreciated.
Cheers.