How can they do it

Hey glow. So as y’all know if some of you may know I have a daughter whose dad is deceased. I know for a fact my baby belongs to him so that’s not the question. My question is how can they do a dna test to prove dna if he in fact dead. He was cremated he wasn’t buried or anything. And his dad is definitely not going to get swabbed for the test. My bd mom sent me this which i have no problem with because I have nothing to hide. What is the process though of a dna test after the father die