Is someone else logging onto my iPhone with another device??

No one can log onto your phone from another device, however, if they know your Apple ID and password and your account does not have 2 factor authentication they can log in to your Apple ID. To find out go to Settings and tap on your name or Apple ID at the top. The next screen will have a list of all devices sharing your Apple ID. If there are any that shouldn’t be there tap on them and you can remove them, then change your Apple ID password→Change your Apple ID password - Apple Support

No.

No, you cannot log in to an iPhone. They can logon to your Apple ID but I assume that you have kept your login information private