First you need https://www.youtube.com/feeds/videos.xml?channel_id , and just paste the channel ID after it,that’s the link for RSS.

To get channel ID: visit the channel you wanna subscribe,then click more to view the full profile,scroll down and click share channel ,there will be a option Copy channel ID

I found it from a blog post from Taiwanese,his example: https://www.youtube.com/feeds/videos.xml?channel_id=UCVXstWyJeO6No3jYELxYrjg