For the entirety of its final season, Supernatural had been invested in the Winchesters gunning for ways to kill God, ending with Billie the reaper-turned-Death resurrecting Jack from the Empty and fashioning him into a bomb that would annihilate Chuck and Amara. But just as Jack begins his transformation in the latest episode, not only is it revealed that Billie had been cooking her own scheme in the guise of helping the Winchesters but also that everything, up until the moment Jack is about to burst into oblivion, has been all orchestrated by God himself.
In Supernatural Season 15, Unity Dean and Jack meet up with Adam, the first man God ever created, to complete the final step of Billie's plan while Sam and Castiel stay behind to come up with a plan that won't need Jack to die. Adam gives Jack a piece of his rib that has the power to create life itself and on consumption, it will fuse Jack's soul with his grace, forming a "metaphysical supernova" so powerful that he will be a "living black hole" for divine energy thus killing both God and Amara.
Sam discovers Billie's true plan.
Wanting to read Chuck's Death Book for himself, Sam finds the Key to Death's library. Here he encounters the Empty's Shadow Ruler who is looking for Billie and makes a major revelation about the Grim Reaper's plans-- her real agenda behind gunning to kill God is so that she can self-promote herself to become the new Almighty. But the power grab isn't her only plan as after becoming God, she plans to employ a strict "Things Should Be In Their Proper Place" law which would mean "everybody going back to where they belong," starting with Eileen dropping dead, again.
Soon after Amara, after trying and failing to reason with Chuck, locks him with herself in the Winchester's bunker as she is under the impression that Dean (Jensen Ackles will bind God. On their way to the bunker, Dean and Jack come to know of Amara and Chuck's presence at the bunker and the latter proceeds to consume Adam's rib. He had been saving till the last minute as he was told that once consumed it will trigger a chain reaction so powerful that there won't be much time to waste or to go back on the plan.
But Sam, now convinced that the plan to blow up Jack to kill God and Amara is beyond flawed, puts himself between Dean and the door of the Bunker. He tries to reason with Dean who pulls his gun on him, teetering on the edge of shooting him as he is willing to trade anything to end Chuck allowing him to reclaim his free will and for a moment it seems that "anything" includes Sam as well. But meanwhile, God is explaining a horrifying plot twist of his own to his sister.
God and his real ending for the Winchesters
After shocking his sister into silence with the disclosure that Dean plans to kill her along with Chuck, he drops another bombshell revelation- everything until the point he is talking to Amara in the Winchester's Bunker has been all orchestrated by him. His true ending for the Winchesters hinged on Sam's drive to know the truth and Dean's rage, his sense of duty that demands to get things done, no matter what the cost.
He knew all along about Jack being alive and just because he couldn't get his hands on his Death book doesn't mean that he can't influence the outcome. All he had to do was plan a few visions like Adam's angelic girlfriend getting the vision that Jack is the one to kill God. He "goaded" Billie a little while "messing" with a few outcomes to bring to Winchesters where they are-- Dean is about to kill Sam, only to realize later that Jack's death would be pointless as well as him being powerful enough to destroy God was all a farce crafter by Chuck.
But just as Chuck convinces Amara to allow him to reabsorb her so as to create a balanced world, Sam manages to get through to Dean and make him see reason. This angers Chuck who is incensed that while all his other versions of Castiel, Dean, and Sam did what he planned for them to do, these on Earth Zero are just too "stubborn and stupid" to be obedient. So, now all the Winchester's are left with is an about-to-blow Jack who Chuck gloats will die before the brothers as they will not be able to do anything to save him.