From zdnet.com
iOS 15 is a treasure trove of new features to explore. One awesome new feature could help prevent you from losing your iPhone.
Called Separation Alerts, what this does is that it can alert you if you leave one of your devices behind by sending a message to other devices that you have with you.
This feature is part of the Find My app, so to find it you need to make your way into the app.
Once there, tap on Devices at the bottom. Tap into a device and look under Notifications for Notify When Left Behind (note that not all Apple hardware currently supports this feature).