I have ublock origin on firefox and it's really bad for me currently. This has traditionally been the good combo I believe.
Not just slowing down, but stopping, then restarting after skipping a few seconds that you cannot access no matter what.
For now the best solution I've found is to copy the video url, open potplayer and just hit the paste command and the video runs flawlessly.
So they'll have to close that loophole eventually, which means enshittifying the video streaming protocol for everything that isn't the native web viewer, which will inconvenience more people who were used to something working, leading to another workaround, leading to...
Youtube is gradually accelerating their enshittification. I'm looking forward to when it comes to a real head. Too many serious interested parties rely on it. I don't know if peertube will be the first fallback, but I'm sure it'll get a big bump.
Some probably use an API of some sort, because SmartTube for Android TV even synchronises your watch history, subscriptions, you login with your google account etc...
Plex does not recognize them in terms of pulling down metadata but you can still organize them in folders and browse that way. I find the Plex route is a healthier way to engage with video content than platforms that just keep serving you whatever the algorithm thinks will keep you peeled to the screen. It's more intentional and less of a passive consumption kinda thing.
tl;dr: Yes, but probably takes some effort for most content.
Plex will play the files, but metadata is hit or miss. If it's something that's on thetvdb or themoviedb, it can be matched as a series or movie, respectively. With some effort, you could also probably include all the relevant metadata when downloading the videos, then have plex use local metadata, which could cover anything not big enough for the big metadata providers.
I think it's also possible to find plug-ins/scripts that will pull metadata directly from youtube, but I've had bad luck relying on that stuff and then development stopping, so I avoid it these days.