Click the magnifying glass in the top right corner next to your notifications. Make sure you are searching for “All”. There appears to be some sort of bug where if you search for communities that it won’t return anything. If you search “All” it’ll find the community. Make sure to put in the actual name of the community you’re wanting to find in this format: !community@server.url. You wan’t to put in the actual community name, not the friendly name. In this example I’m searching for Youtube News (yt_news) on Lemmy.link.
Every community has their feeds listed in the sidebar. That is actually how the bot is controlled. It looks up communities, scrapes any RSS feeds in the sidebar and then posts any unseen links to the community.