You can’t login on another instance, but you can interact with them from your own instance.
See it as email: I could be using Gmail and you could be using Outlook, but we can still interact with eachother. We cab send emails to eachother. But when I want to send you an email I’m not going to use the Outlook website because you happen to use Outlook. I use Gmail and so my account is on the Gmail servers. So I login on the Gmail website to send you that email.