Just passing on a tip from FediTips I didn’t know about. Using the format
(server)/tags/(hashtag without the #).rss
Your RSS Reader can follow hashtags e.g.
https://mstdn.social/tags/dogs.rss
You can see everything with the hashtag ‘dogs’ on mstd.social and every instance mstdn.social can see.
Feed for this Community is
https://lemmy.ml/feeds/c/fediverse.xml?sort=Active
you can change the sort to suit you.