Based on my experiments, being new to this as well, it feels like each site offers the ability to create credentials, but gives you access, via their particular interface to the whole network of lemmy sites that are sharing with one another. I'm on via lemmy.world and using the Jerboa app, so I think If you can find the post on beehaw again you should be able to comment from there.
In googling around a bit I found someone made a nice analogy to email where everyone can be on different sites, but still communicate between them.
Another thing I learned from this is that apparently not all sites are necessarily connected, but that feels like it defeats the purpose.
Probably not bad. I tried it too earlier just to see, and just stumbling around as well.
The app has helped me a bit in that it seems to hold some of the context for my account. Meaning as I move around these communities it keeps me in my context where it knows which site I'm connected via.
Links have been troublesome for me thus far though. Opening the other communities in their native interfaces instead of in the app or somehow via the site I'm logged into. Probably something to ponder on as development continues. Maybe someday the app can handle the "magic" of getting a post via a link to another site within my site's context if they are connected.