It is a pain - I've found 3 formats for users, channels and playlists:

    https://www.youtube.com/feeds/videos.xml?user=<user name from url>

    https://www.youtube.com/feeds/videos.xml?playlist_id=<id from url>

    https://www.youtube.com/feeds/videos.xml?channel_id=<id from url>

I wonder if there are more?

Sorry for the repeated post, but if you have an account, you can subscribe to the channel and then get an OPML file with all the feeds from the Subscriptions Manager page.

One quick way to do that is by using NewPipe Android app. It has an RSS feed button. Plus, you get a lightweight YouTube frontend.