Earlier quoted context omitted.
I totally agree. The idea of running a server application in Docker and then firing up a browser for something like this just seems so... round-about and alien to me! Is this the direction software development is really going?
For a large group of people, the vast majority of their computing and consumption is done apart from a desktop or laptop. Something like this makes perfect sense in a mobile-first world.
Besides that, Invidious has a download button for audio and video.
As far as apps there's very simple mobile apps: Tubular and newpipe both support downloading, and ytdlnis and seal are explicitly for downloading. I'm sure there's a dozen more in F-Droid.