How do I connect to other fediverse things?

I am pretty new to the fediverse, but have been sort of aware about its presence since elon took over twitter. I am having some trouble figuring out how to use other parts of the fediverse. For example, I came across a link to a post from beehaw, but was unable to comment on it. How do I do that?

Also, I have heard that I can use a single login for a fediverse service and use that across all the different communities on the fediverse, such as logging in to mastodon using my lemmy.world credentials.

Thanks for the help!