I booked a business class trip from the U.S. to Germany on United with a short layover in Frankfurt where I change to a Lufthansa flight to Berlin.

Is it correct that I cannot check in or obtain a seat assignment on the Lufthansa flight until 23 hours prior to the flight?

Also, can the United confirmation number be used for check in on the Lufthansa flight? Or is it necessary to contact Lufthansa to obtain a confirmation/booking number to check in and obtain a seat assignment?

Sorry for the newbie question, but this is the first time I have encountered this situation.

Thanks