I don't know what happened at the end of the novel but in their previous lives, after the two years more than a thousand degree celsius temperature happened, a moment of peacefulness arrived and they thought that the apocalypse already ended but the low temperature began to happen. That's the time when the previous life MC runaway from the previous life ml.

The ML obtained great power at the price of madness, during which he would abuse the FL. She escaped him but died. In remorse, he sacrificed himself to give her a chance at rebirth.

She heals him, so that he never descends into madness and the first life is averted. Instead, he becomes a loving, spoiling, domineering-CEO type and follows her around. And because he is the strongest person, and is also able to stay sane in this life thanks to her powers, the entire post-apocalyptic world is a safer, more civilized place than the first life.

the plot runs faster and is written in broader strokes until the end. I would have preferred more detailed and slow progression about how they survive the final days until the apocalypse is over, and more storytelling about the entire cast of characters.

The protagonist is a girl who lived through roughly 2/3's of the apocalypse, as an ordinary person, and the girlfriend (read: pet) of the ML. He protects her and keeps her safe and fed, but not happy, she wants her freedom. He is very sick mentally and physically from years of practicing his ability (which he and a group of other 'cultivators' have inherited for generations, and which damages his body.), so he is unable to be his best self but is genuine in his emotions for the FL, it just comes out in a so extreme way that the FL lead feels that he is insincere. After being kept by him for 2 years, she stabs him with a fruit knife and escapes but dies outside within days because she was unprepared for the 3 part of the apocalypse and is an ordinary person. She freezes to death.

Then she wakes up again. And this time she is preparing for the apocalypse, she announces she is going back home for a vacation but really she is shopping for supplies and building a base for her grandmother and their fellow villagers. Mostly old people, women and children.

SHE HAS A CHEAT:

In her previous life the ML gave her a necklace (later revealed to be an old family heirloom), which she presumably died wearing, because she doesn't mention taken it off before fleeing (and dying) and when she is reincarnated she is still wearing it.
The necklace is broken but inside the necklace is a 'gourd', which is really an magical space, in which she has a magical spring and which she can grow plants in (but not rear animals). Inside she finds several ginseng, among which is one several hundred years old, which she uses to fund her purchases.
She can only stay in there for 2 hours at the time and usually spends that time tending to plants and once or twice to hide in.

And lastly she pulls herself and a classmate/friend of hers out into an 'golden rain' the first day which awakens an ability for both of them so in this life they both have abilities.

also this friend is one of the few people she actively tries to help other than her grandmother and their village.
She mentions several times that she selfishly only cares about their fate (which is in my opinion all right, since as she says pretty much everyone in the apocalypse thinks like that).

That's it for my maybe not so spoiler-free summary