My iPhone was stolen. Who to contact if I have AppleCare?

You can file a claim here - File an iPhone Theft and Loss Claim - Official Apple Support

Note that you need AppleCare+ (not the one-year AppleCare Limited Warranty) with Theft and Loss Coverage.

AppleCare and AppleCare plus does not cover lost or stolen devices.

you would have to have purchased AppleCare plus with theft and loss coverage which includes a insurance policy.

If you purchased it, you need to file a claim with the insurance company:

File an iPhone Theft and Loss Claim - Official Apple Support