how do I use "find my phone"

Apple - Find My iPhone, iPad, iPod touch, Apple Watch, Mac, AirPods, or AirTag - http://www.apple.com/icloud/find-my

Use Find My on iCloud.com - https://support.apple.com/explore/find-my

or go directly to: https://support.apple.com/en-us/guide/icloud/locate-a-device-mmfc0f2442/icloud - Locate a device in Find My iPhone on iCloud.com - Apple Support

Use the Find My app to locate a lost or stolen device - https://support.apple.com/HT210515

Find friends and share your location with Find My - https://support.apple.com/HT210514

Get help using Find My Friends https://support.apple.com/HT204407

There's more if you want them, or if you would care to be more specific in your question... ;-)

Start here: https://support.apple.com/en-us/HT210515