How can I find out who is connected to my phone
How can I find out what devices are connected to my Apple ID and my phone number
iPhone 8 Plus
Question marked as
About your Apple ID you can see in Settings > Your Name at the top, roll down and see the list of devices using your Apple ID.
Phone number has nothing with it. It's just a number and if it's assigned to your SIM card and/or e-SIM, that's it.