I have an app for my iPhone that lets me record my screen, might be a better way out there, but i just activate record on my screen and then let the video play through.

I use youtube dl personally. A grpahic interface version cuz the command prompt version was giving me a headache. It's a bit technical, but here's a reddit sub for it https://www.reddit.com/r/youtubedl/ .

Otherwise google around and find some websites that'll do it for you.