How does following people on kbin work? Do I get shown their activity somewhere? Only boosts or other stuff too? And where?

In theory, anybody you follow you should see their new threads in Threads on the Subscribed view, and their new microblog posts in the Subscribed/Microblogs view. I am not convinced it actually works for Threads - and it may not work for microblogs either.

Certainly following Mastodon users shows their posts under Subs/Microblogs.

Following Lemmy users should show their posts in the Threads view (as above, not sure if this works).

Boosts should show up as a new post "boosted by" in either Threads or Microblogs depending on what it originally was - but this appears to be unimplemented.