You will find the PUK for the German identity card, the residence permit and the eID card for EU citizens in your PIN letter. You can find more information about the PIN letter here.
You need the PUK if you have entered your ID card PIN incorrectly three times and as a result it is blocked. You can use the AUTHADA app and the ten-digit PUK to unlock the PIN yourself. To unlock it, select the “Unlock PIN” function in the AUTHADA app. After you have successfully unlocked your ID card PIN with the AUTHADA app and the PUK, you have three new attempts to enter the correct PIN.
If you have forgotten the PIN or no longer have your PIN letter, you can also have your PIN reset without the PUK. You can find more information on resetting the PIN here.