Yes, all the services share the same underlying protocol and share content.
If you go to your frontpage and switch from “local” to “all” you’ll see posts from many different sites and services.

You already posted this from sh.itjust.works to lemmy.ml, so you’re already doing it.

Regarding kbin, word is it’s really struggling under exodus and their Cloudflare DDOS protection might mess with federation.

That said, I’m able to read 2 magazines using Jerboa, maybe they’re the only ones federated so far with my instance? I haven’t figured out how to “discover” unfederated kbin instances. /kbin meta@kbin.social and kbin@kbin.social They might show up if you type “kbin” into the community search bar in Jerboa if your instance knows about them.

Please correct me if any of this is wrong, I’m just throwing out my current understanding of the situation.