If software on the fediverse such as lemmy and kbin are connected similar to emails, how come I cannot use kbin with my lemmy account and have to create a new one for kbin specifically? Is this just a bug, or have I misunderstood how the fediverse works?
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.