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?