PieFed help

Pretty much, yeah.

Topics are the administrator curated categories of communities, intended to help people identify communities they may want to join/subscribe to.

Feeds are exactly as you say, user curated categories of communities. Public ones are someone's shared collections/curations that you could join/subscribe to if it looks like something you're interested in and don't feel like making yourself.

Yes: Topics are basically pre-built, fixed Feeds, whereas Feeds are user-customizable and also shareable Topics, and subscribing to a community doesn't affect it showing up in one of those (at least, definitely not Topics, and I presume therefore also Feeds).

The learning curve here is real yo, but otoh it is quite useful to have such choices. e.g. if you don't want politics flooding your Subscribed feed, then don't subscribe to any, leaving you free and clear of it - and yet all you have to do is click the News & Politics Topic and you can see it all, thus allowing you to have your cake and eat it too (both avoid politics, or any other high-volume communities, and yet visit them too, anytime you want).

Another method is to click the bell icon next to the community, topic, or feed (or user, post, comment, pretty much anything) and then you'll receive an actual notification whenever something is posted to it. Feeds are better, e.g. if you didn't check PieFed for a week it wouldn't flood your inbox with posts, but for low-volume communities that's yet another way to get at them.

Feeds are brand-new, so they might end up being reorganized somehow and integrated differently than they are now, side by side along with Topics. Hrm, maybe Feeds could go higher in a combined menu, so that as people subscribe to Feeds the Topics get pushed down? Or maybe people don't need Topics at all, now that Feeds exist, so those could just become pre-subscribed-to Feeds, which someone could leave if they don't want them anymore, and want their Feeeds to appear higher. Lots of thoughts to still think through there...:-)

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.