My understanding so far after a couple days is that joining a community gets it to show up on your front page but has no effect on it displaying under topics.
In topics these are pre-built groups of communities (like multireddits). You can’t modify them.
Feeds are custom groups (like a multireddit) you can make that are just like topics except under your control. You can manage what communities are in the feed and show up under each category. You can create a feed that shows up as a “sub-feed” just like in topics. If you join a public feed it is like a user made topic where you can’t change which communities show up in the feed.
Am I understanding these correctly?
You can also copy a public Feed to make it your own, if you want to amend it.
There’s some overlap between Topics and instance-wide Feeds. Hopefully, Topics can be removed one day, so instances will just have Feeds - some instance-wide (created by the admin), and some public (created by users). This will make things less confusing. Topics are currently used for new-user onboarding though. There may also be other reasons for a reluctance to remove them, I’m not sure.
As a new user I latched onto the topics pretty quick and used it for navigating around. I will say despite the confusion the experience here is 10x better than on the lemmy instance I created an account on. I will work on Feeds over time as I navigate around and see content I like.