Thank You Letters Destination Letters How Does USPS First-Class Mail International Get To Its' Final Destination?

How does USPS First-Class Mail International get to its' final destination? - thank you letters destination letters

To say that I received a letter from Philadelphia to Germany chic.


I know it will be by air, but he is booked on a flight or not taking place? And you go on a flight to Germany or France could be transferred and surface mail to Germany?

How the system works as a whole?

Thank you.