Need help with getting a birth certificate

My grandmother was a German citizen born in Schrimm, Posen in 1941. We do not have a copy of her birth certificate, but we do have expired German passports and ID cards. I am unsure if I need to go through a Standesamt or Poland to request her birth certificate from since one of her ID cards says “Schrimm/Warthegau,” which I understand to have been part of Germany at the time. Does anyone know how to proceed?