my cousin iphone 12 is stolen

If Find My was enabled, follow the steps here - If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

If Find My wasn't enabled, you can't track it in any way. Contact your carrier and ask them to disable your account, and report it to local law enforcement.

If Find My iPhone or Find My iPad isn't enabled on your missing device - Apple Support

They can only do that if they know the password. It's more likely that the device is powered off. For FindMy to work the device must be powered, connected to a network, and FindMy must be enabled. See: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Your cousin will have to log into FindMy (iCloud.com/find) to find their device.

We're not Apple employees- just volunteers helping others out on our own time.

Additionally, an iPhone cannot be tracked with the IMEI or serial number. Only Find My can do that for you.

No one else can track the phone. The ONLY way to track the location is using Find My.