As a fellow former redditor who has figured this stuff out (kinda) in the last week here's my best gist of the situation:

Lemmy/Kbin are like reddit style interaction

Mastodon is Twitter style interaction

All three use the "ActivityPub" protocol, so all three can see posts and comments from all three systems.

Additionally, each system (Lemmy, Kbin, and Mastodon) can be hosted on "instances".

Instances are like "servers" that can be linked to other instances.

Talking about Lemmy/Kbin first:

You can pick a "home" instance and sign up for an account there (like you did with kbin.social). You can subscribe to all their local communities (like subreddits).

On Lemmy, you can also go to the "communities" page and then click the "all" tab" and you'll see all the communities from not only the local instance you're on, but it will also have links to all the communities from all the other instances that your instance is "federated" (linked) with. And you'll be able to subscribe to those communities as well.

What if you don't see the community from the other instance that you want to join? Just go to that other instance, grab the URL of that community, copy it and paste it into the search bar of your home instance, and that will create a link where you can click on it and subscribe to it.

To see what instances your instance is federated with, click the "instances" link in the bottom-right of every page.

On kbin, it looks like you have "magazines" that you can subscribe to. The search function behaves the same way as well.

Mastodon - if you're not into twitter, don't bother. But basically, it's twitter setup the same way, with instances instead of a server, and then you can follow other people from that instance or from other instances.

Mastodon also treats Lemmy/Kbin communities as if they're "people" so you can follow lemmy/kbin communities and comment in them using the twitter-like interface.