User : https://www.youtube.com/feeds/videos.xml?user=<XXX>
Channel : https://www.youtube.com/feeds/videos.xml?channel_id=<XXX>
Playlist : https://www.youtube.com/feeds/videos.xml?playlist_id=<XXX>
User : https://www.youtube.com/feeds/videos.xml?user=<XXX>
Channel : https://www.youtube.com/feeds/videos.xml?channel_id=<XXX>
Playlist : https://www.youtube.com/feeds/videos.xml?playlist_id=<XXX>
https://www.youtube.com/user/mkbHD
https://www.youtube.com/c/mkbhd
For some reason though using channel_id=mkbhd doesn't work. This is a working link: https://www.youtube.com/feeds/videos.xml?channel_id=UCBJycsm...
It is embedded on the channel page as <link> element, your RSS reader should accept the web page link and figure it out.
"/user/"
"/channel/"
"/playlist?list="
I think the newer slug format for channels is "/c/" in which case I think you do have to extract it from the HTML source, but they now include the feed URL anyway.
<link rel="alternate" type="application/rss+xml" title="RSS" href="FEED_URL">