This has been my experience playing with the microblog section. Somebody please feel free to correct anything I'm wrong about here.
Do I follow mags? How?
The Lemmy magazines you subscribe to will also be subscribed as hashtags on the Mastodon side. So if you're subscribed to /m/technology, you'll see Lemmy threads in the magazine as you normally would, but you'll also see anything posted on Mastodon with the #technology hashtag. From any user. So if I were to go onto a Mastodon server and post something with that hashtag, you should be able to then see it under the microblog section of the /m/technology magazine.
Do I follow people? How?
You can open any user's profile from the microblog as you would from a Lemmy thread. I believe that subscribing will put them into your Hot and All microblog feeds when viewing the microblog tab from the main page.
My feed includes all sorts of items not just who and what I follow. Why?
They're showing up because they posted something with a hashtag that matches a magazine you're subscribed to.
When I post, where does it go? Who can see it?
Your posts go to Kbin's own Mastodon server. This means that the people who will see it are Mastodon/Kbin users who have subscribed directly to your Kbin account, Mastodon/Kbin users who are viewing a hashtag mentioned in your post, or Kbin users browsing the All microblog feed around the time you post (I don't know if Mastodon users can view Kbin's local feed; there's probably a way but I have no idea how that part works). If you post to a magazine's microblog, this is as if you were to write a microblog post using that magazine as a hashtag (although it will be a "hidden" hashtag in the post).
and anything else because I don't know what I don't know.
I've not tested this out, myself, but I believe that if you write a microblog post to a magazine, such as /m/technology, then that "hidden" #technology hashtag is still searchable by Mastodon users. Though I've not tested it.
Also, upvotes in the microblog section act as Favorites in Mastodon, and boosts are still called boosts in Mastodon, but are more of a "retweet" function. So boosting a post will appear to a Mastodon user as if you were retweeting their post. There does not appear to be any equivalent "quote retweet" function, as this was largely used for bullying on Twitter.
Also, some clarification on terms: On Kbin, "microblog" refers to the Mastodon element of the site, tweets/toots are referred to as "posts", whereas Lemmy threads are "threads". "Replies" are replies to microblog/Mastodon "posts", whereas "comments" are replies to "threads". Just in case you're browsing anybody's profile and are confused by the differentiation between these lists.