How do I find out if an unauthorized device has logged into my Apple ID?

When I opened my computer a few minutes ago, a message came that someone a new iPad had been logged into my Apple ID. The only devices shown on my account are my own -- none new. How do I find out if someone else is using my Apple ID?

A couple years ago, during the Covid shutdown of many services I took my computer into the nearest Apple for service. They sent it to some other service agent who fixed the issue BUT also put in a different hard drive for some reason I've never understood. Since then every so often, there is confusion with Apple ID, some Apple Apps (especially Books, and some websites saying this computer is not recognized. So I'm wondering if that might have something to do with the "new iPad logging in" message.

How do I find out if an unauthorized user has breached my account?