How can I find my offline iphone

This support article describes your options for an offline device If Find My is offline or not working - Apple Support

Generally you can’t … althogh Apple does provide “Offline Finding” for a short period after a device had been turned “off” AND several other conditions are met.

Regardless, using FindMy (either the app or at https://icloud.com/find) is the ONLY Apple-provided method available.