Kbin and Lemmy are both different pieces of software that can be used to spin up an instance of a reddit-like news aggregator. Both speak the ActivityPub protocol, a common “language” that allows instances in the Fediverse to connect. In theory a Lemmy or KBin instance can connect to all other Fediverse instances via ActivityPub, no matter whether the server runs on the Lemmy or KBin Software or it’s an instance of Mastodon or Pleroma. But the administrators of a particular instance can restrict which servers they want to federate with, but that is not a feature of the used software. A caveat is, that in practice there had been some issues with federation between Lemmy and Kbin servers in the past, at least I heard so. You could regard this as software bugs and in current versions that should be better.

Also a reason that many people choose KBin over Lemmy is that supposedly, the original Lemmy developer has some far-left “tankie” world-views and runs the “lemmygrad” instance. Many instances defederate with that (and maybe other Lemmy instances), but those are social aspects, and not an issue with the Lemmy software itself.

I often share (“boost”) lemmy posts on mastodon. I would like to be able to add hashtags to the boosted post, because on mastodon I rely on hashtags to find content that interests me (e.g. I follow certain hashtag. Lemmy doesn’t need those because the general theme and topic is often obvious from the community context it was posted in, but this context is lost when sharing on mastodon. For example, when I post something in c/[email protected], for lemmy users it will be clear that this post is about Baduk (the Korean name for the game of Go). But when I boost the post on my private mastodon, it’s not obvious anymore that this was posted in a Baduk community and the Baduk-interested people on mastodon will never see the post except if they follow me or the lemmy comunity. All solutions that come to my mind seem a bit awkward, are there any best-practices for that?

Comment threads are collapsible afaik, just touch and hold onea comment and release again. Admittedly there are no visual hints that this is possible, there a tutorial or something like that might help.