Two Apple ids same IMessage

First I’d say this is exactly what I wanted when I set this up but how the heck did it happen? I’ve got my iPhone 15 and an iPad Air 5 gen I just bought. I bought the iPad for school purposes only, I even created a separate Apple ID so my “school” iPad can remain distraction free from different notifications. But… I wanted IMessage on the iPad from the iPhone messages. Somehow on the list of send and receive my school Apple ID is now on there and able to get messages from my other Apple ID. HOW??? I don’t want this to change but I would like to understand how and why in case I need to do it again. Side note I did setup family sharing and bought the iPad with the Apple ID associated with the iPhone.