In actuality, the Last Son of Krypton stole Superman's body and put it in a regeneration matrix in the Fortress of Solitude, drawing on his recovering energies to power himself, as bright light blinded him.
How does Superman come back to life after the death of Superman?
The regeneration matrix allowed the Eradicator to siphon off Superman's powers for himself in order to pose as him as it slowly revived him from death, or at least some kind of special Kryptonian death-like state.
How long was Superman dead before they brought him back?
It can be assumed that not more than a month occurs between Steppenwolf's first attack and the resurrection of Superman. Based on the time that passes between the two movies and within Justice League, Superman was buried for about 2-4 months before being resurrected.
One such Box was able to resurrect the deceased Superman when combined with a Kryptonian Genesis Chamber in Scout Ship 0344 and a massive electrical charge from the Flash. A Mother Box was also able to stabilize the critical condition of Victor Stone, saving him from near-death.
The Death, Resurrection, and Immortality of Superman
We know from comics, like the iconic storyline for Death of Superman, that he can indeed die, too. In that continuity, he was famously killed by Doomsday, with his soul even leaving his body and moving on to have adventures with DC's mystical characters.
Whether Superman can truly die is something that has never really been answered. We have seen him die and come back but we have also seen him become immortal.
The Eradicator told Clark that he actually died and was brought back only by luck and Kryptonian technology. Others stated that Superman had just fallen into a healing coma and could have been revived by sunlight (which he ironically received when Cadmus stole his body and experimented on it).
Just like in the comic books, Superman had died following a devastating and brutal battle with Doomsday. But while Clark Kent was brought back to life thanks to a Kryptonian regeneration armor in the source material, the movie's return played out much more differently.
Why did the Mother Box activate when Superman died?
That sound goes out into the world, echoes across the globe, and if there were sleeping Mother Boxes on Earth, that sound would wake them up." Once awake, the Mother Boxes relayed the crucial information of Superman's death back to Steppenwolf: Without the Kryptonian to protect the planet, the Earth was safe to invade.
Meanwhile, four men claiming to be Superman—Steel, the Cyborg Superman, Superboy, and Eradicator—emerge, and Lane discovers his grave is empty. Steel and Superboy are disproven as the original Superman, but the Cyborg and Eradicator both seem to recall some of his memories.
Kal-El / Clark Kent / Superman - Can vary living into his hundreds, thousands and beyond. General Zod - Would've be able to live well into his hundreds. Faora-Ul - Will be able to live well into her hundreds. Nam-Ek - Will be able to live well into his hundreds.
Superman, already weakened by kryptonite exposure, is fatally wounded by the creature before it dies. Following Luthor's arrest, Batman confronts him in prison, warning him that he will always be watching. Luthor gloats that Superman's death has made the world vulnerable to powerful alien threats.
He takes the full force of the bomb, but the sun revives him and he returns to the fight. Superman picks up Batman's Kryptonite spear, and stabs Doomsday with it. Doomsday stabs him too and Superman is weak enough from all of the various stabbings and explosions that Doomsday's stab proves fatal.
Batman feared Superman's unchecked power, and Superman (Henry Cavill) hated Batman's brutal methods, but the real reason why Batman and Superman fight is manipulation by Lex Luthor (Jesse Eisenberg). Batman emerges victorious, and from a narrative perspective, Batman v Superman could go no other route.
Snyder's intention for the dirt rising on Clark's coffin to be a symbolic ending might not have been noticed by all, but it does serve multiple purposes. Even though the ending's primary function is symbolic in Snyder's mind, it also works as set up for what else he had planned.
The Blue orbs merge with New 52 Superman and Lois Lane, and they both remember Jon Kent. There is a huge blue light and the timelines from New 52 Superman and Pre-Flashpoint Superman merge to form one new timeline with everyone regaining their memories of what happened before Flashpoint.
Does Superman come back to life after being killed by Doomsday?
To make a long story short, Superman and Doomsday fought to the death, before the Kryptonian Regeneration Matrix zapped Superman back to life after the Eradicator stole his body and brought it to the Fortress of Solitude. And all of it was thanks to a wedding needing to be postponed for the sake of marketing!
Batman v Superman: Dawn of Justice killed Superman, who returned in both versions of the Justice League movie. Now that he's back in action and shown among the living, it also needs to be explained how "Clark Kent" is back.
[Film/TV] After Superman is brought back in Justice League, wouldn't someone notice Clark Kent also came back from being reported dead in the same timespan and put two and two together?
He also regenerates his body in Batman: The Dark Knights Returns when (like Wolverine in the past) a bomb leaves him as a shell of his former self. So, it's safe to assume Superman can heal himself, but moments like the one above show he's got the ability to regenerate.
While Thanos might have the brute strength to defeat Thor, for example, it's a different matter when it comes to the Big Blue Boy Scout. That's because Superman remains much more powerful than Thanos — meaning that, in a straight fight, the former is bound to take the latter down.
Green kryptonite weakens Superman and other Kryptonians, and is fatal with long-term exposure. Kryptonians under green kryptonite's effects experience severe muscular weakness, usually to the point of collapse, and excruciating pain, with both conditions progressively intensifying.
This means that once exposed to a yellow sun, Kryptonians will age normally up to their prime, but the aging process will slow down considerably when their bodies reach a peak level of maturity. They are not immortal per se, but they age so slowly that they may as well be.
