Using iPhone in germany

As long the iPhone is unlocked, it should be usable with a German carrier, in Germany.

How to unlock your iPhone for use with a different carrier - Apple Support

Note that the warranty if any will only be valid in the country of purchase. In this case the U.S. So if anything happens to the iPhone that requires warranty service, you will need to take it back to the U.S.

Apple does not take international shipments for service.

Yes, as long as the phone is compatible with German frequencies.

Obtaining iPhone Service in the European Economic Area (EEA) and Switzerland

Wireless carrier support and features for iPhone in Europe