An iPhone can be tracked using Find My. It cannot be tracked by its serial number or IMEI.
An iPhone can be tracked IF Find My was enable, it is powered on and connected to a network. Thieves are smart and usually place an iPhone in a Faraday Bag, which prevents it from being tracked.
See this link on what to do if your iPhone was stolen --> If your iPhone or iPad was stolen - Apple Support
And this about how to try to track it --> Find your lost iPhone or iPad - Apple Support
Be advised that Apple does not get involved in lost or stolen devices. You may receive a message, purporting to come from Apple saying your device has been found and all you have to do is validate your sign in credentials. DON'T FALL THIS. It's a scam to try to get you to give up your credentials so they can wipe the phone. The thieves, sometimes resort to threats. DON'T FALL FOR THIS EITHER.
Finally, contact your carrier and ask them to Blacklist your phone, so it can't be activated on any network.
The sad reality is the vast majority of stolen iPhones are never recovered.