@notjustbikes theres’s two services:
Lufthansa Express trains, with the train technically being „a flight“ (designation, signage etc). You literally check your luggage on the train and it’s automatically routed to your destination through the airport, onto the aircraft etc. You get a single ticket.
And Rail&Fly: super convenient for people traveling to and from all airports in Germany and usually included in any fare booked through popular travel agencies: https://www.bahn.de/service/buchung/bahn_und_flug/rail-and-fly-english