Cross-device notifications between iPhone 11 Pro Max and iPhone 16 Pro Max

My husband got a new 16 Pro Max and now he is getting notifications from websites that I am visiting with my 11 Pro Max? I scanned a QR code with my phone but he received the notice about scanning it. I also clicked on information from an email I received, and he got an email saying, “Thank you for you interest.” What is happening?

[Re-Titled by Moderator]