That’s not how it works.

You cant log into hotmail.com with your gmail.com.

You can however use your gmail.com account and talk to the hotmail.com user, by sending an email.

You can look/browse other fediverse services content, your can star/like it, comment on it.

But you can’t log into that service.

You have misunderstood. You can’t use your gmail account to sign into an outlook account. However, you can email people with the same or different email providers.

If you create an account with Gmail, can you then login to Yahoo! email and send emails with that same account, or do you have to have a Yahoo! email address to send emails from their website?

You can send an email from an @gmail.com email account to an @yahoo.com email account, and vice versa. But you cannot send an email from an @yahoo.com account on the Gmail website.

You can’t use your Gmail account with let’s say Hotmail. I think that’s the analogy very commonly thrown around here - I’m not sure how accurate it is though

I’m using an account registered on lemmy.click, and am subscribed to magazines from kbin.social. They show up in my stream, and I can use them as if they were lemmy communities.

Conversely, I’ve seen kbin users commenting in lemmy threads.

I don’t know how much of this is the norm or the exception. I heard that kbin.social federated with lemmy, that’s when I went looking for their magazines.

But generally speaking, you don’t have to create a kbin specific account and can access kbin with a lemmy account. In practice, the scope might still be limited. I also still have a lot to learn.