Military records of your German ancestors can be illuminating, though it is very hit-or-miss what records survive. You can order military records using the forms at the Bundesarchiv.
We sent a request to poststelle-pa@bundesarchiv.de in Deccember of 2025, and in April 2026 received an email response with PDF files of all records found. For our great-grandfather's service in WW I, only the index of two hospital stays in 1917 remain.
We later received the bill in the postal mail for a quarter hour charged at 20 Euros/hour, so 5 Euros total. They accept SEPA payments to a bank account number called an IBAN, which we used wise.com to pay for a few cents in fees. The Bundesarchiv also includes instructions of where to send cash Euro notes if you wish, though personally I wouldn't recommend doing that.