"If soul is not separate, how does resurrection work for those who are cremated?"

(The following paragraphs are not in a strict sequence, but I hope they build up a picture.)

Cremated quickly or rotted slowly, it's all the same. Molecules, atoms, "quantum events", death. And we could equally ask, "how can I keep the same identity from one moment to the next?" You may think that's obvious, but you can't answer it except that "God wills it". And anyway, why do protons attract electrons? And an endless stream of such unanswerables. Resurrection is nothing special! It is no harder to understand or accept than everything else.

Psalm 8 asks "What is man that thou art mindful of him?" To "science", he's just a bunch of quantum events like any other body. But the same Psalm continues, "Yet thou hast made him little less than God". Our humanity is inexplicable. We are "fearfully and wonderfully made" (Psalm 139:14).

Genesis 18:14. "Is anything too hard for Yahweh?"

Ecclesiastes 3:11. "He has put eternity into man's mind, yet so that he cannot find out what God has done from the beginning to the end."

Best of all, John 11:25-26. "I am the resurrection and the life; he who believes in me, though he die, yet shall he live, and whoever lives and believes in me shall never die." Or more briefly, 2 Cor 5:17: "If any one is in Christ, he is a new creation." That's "creation from nothing" by the Source of all things. He often builds on what is already there, but can start again if necessary.

How does God work? He has no "how". He is the Source of all "hows".