What apps run in conjunction with the calendar on my phone

My phone has been hacked and now I keep getting notifications thru my calendar that are not anywhere to be found except in the notification and I got a new phone it’s still happening which the sales rep said wouldn’t happen How can we run a check to see if an app is the problem or better yet how do I stop this. I also see some of my save passwords are not on my phone and there are passwords saved that are not from me. I’m the only one with physical access to this phone