Depending on their place of departure, travellers have to provide a negative test result or proof of immunity, and comply with quarantine regulations when entering Germany.
Before entering the country, you are required to register (registration form) and carry the confirmation with you when travelling. This confirmation will be checked by your carrier and potentially also by the Federal Police in the context of the entry control. Moreover, the Federal Police also carries out such border controls when performing its border police duties (e.g. if you enter Germany by road).
Should you, for want of the necessary technical equipment or because of a technical malfunction, be unable to conduct a digital registration on entry, you will be required to fill out a substitute registration. Please refer to the substitute registration for information on where to submit it (e.g. to the carrier or to Deutsche Post E-POST Solutions GmbH, D-69990 Mannheim).