The link you included (
https://www.binwang.me/2022-10-29-RSS-Brain-Yet-Another-RSS-...) has some details, but I guess you are right it's not obvious enough.
The way RSS Brain knows upvotes is by leveraging customized fields in ATOM feed. It will parse the tag <*:upvotes>, <*:downvotes> in each of the article and compute score from there. That means the RSS feed should support those fields in order to let RSSBrain know the upvotes. But the protocol is open so any RSS feed an RSS reader can use those if they think this is a good idea.
So it can only get exiting upvotes from the site. It has no way to upvote when there is no such feature in the original site. However, you can have a customized RSS feed to use upvotes in a different way: for example, for a news website, the position and size of an article usually means the "upvotes" from the editors. Which you can give some weights based on the position and generate a RSS feed that use those weights as "upvotes".
I did two things to make it easier at the beginning:
1. When you subscribe to HackerNews or a subreddit in RSS Brain (that means you paste HackerNews/subreddit URL directly to RSS Brain to subscribe), it will try to parse the content by itself (using some APIs), instead of trying to find a RSS feed. It's not a standard way, but just a way to make it easier while no RSS feeds have those fields yet.
2. I pushed a PR to a very popular RSS generator: RSSHub, so that you can easily add those fields (the doc: https://docs.rsshub.app/en/joinus/quick-start.html#submit-ne...) when create a RSS feed using RSSHub. I've updated its HackerNews RSS feed to generate upvotes fields as well.