Comment by Arnavion

(From that link, about adding new subscriptions)

>The only real trick is that most YouTube channels use a vanity URL and it’s more complicated to get the channel ID in those instances.

Go to the channel's videos page ( https://youtube.com/.../videos ) -> right-click -> View page source -> search for "rssUrl" . It'll look like https://www.youtube.com/feeds/videos.xml?channel_id=UC...

Bonus: Replace the "?channel_id=UC..." with "?playlist_id=UULF..." to get a feed without shorts and livestreams.