The finger snap! That moment in The Avengers movie that had us all in our feelings. Some going into deep depression others refuse to believe all is over!
FanTheory Some one on reddit thought y'all might need to know!
[Avengers 4] What Doctor Strange really did at the end of Infinity War
At the start of Avengers 4, Earth is experiencing the apocalypse, while Thanos is chilling on Titan. The Gauntlet is a wreck, but he can still use it to harness the Infinity Stones - after the Snap, he used them to restore his homeworld to the bucolic paradise he remembers. The Gauntlet might not be capable of another Snap in this state, but restoring Titan shows Thanos can still control the Stones and is still massively dangerous. So it's a surprise to him when Titan suddenly reverts to the desolate ruin it was at the end of INFINITY WAR.
That's part of Doctor Strange's plan. We saw in DOCTOR STRANGE that he can cast a spell on the Time Stone to impose an infinite time loop, represented as a green energy ring rotating around his left wrist. No matter what happens, it always resets to the point in time when the spell was cast. The Stone and the spell on it keep working, even after Doctor Strange gets (repeatedly) killed. This point is important, and ties in with Strange's later reminder to Ebony Maw that spells he casts on the Time Stone will persist even after he dies. When he finally gets Dormammu to relent and releases the spell, the energy ring on his left wrist isn't seen again.
In INFINITY WAR, once on Titan, he sees what needs to be done for the one future where the Avengers win. He knows the Snap is coming, and knows he gets dusted in the Snap. His plan is to trap Thanos in the same time loop he used on Dormammu. But his confrontation with Dormammu was in the Dark Dimension, a realm where the normal rules of time don't apply and presumably, they're easier to muck around with. In our reality, simply trapping Thanos in a time loop wouldn't last, especially with him being the strongest guy in the universe and all. We saw how he bulldozed through the other spells Doctor Strange launched at him, like the multiple avatars and the Mirror Dimension.
So Strange needs a way to bind Thanos himself to the time loop spell without him knowing, even if he zips a million miles away. For that, he needs Thanos to be carrying the Time Stone. Strange keeps it hidden during the fight, knowing Thanos will win (which is why Quill's freakout didn't spoil the plan, but was necessary for it). We may even find Strange was manipulating the fight just to prevent Avengers from getting killed, say by using portals to get them away from Thanos the instant they landed a blow. He needed a believable way to surrender the Time Stone to Thanos, without him catching on. His claim that he was trading it for Tony Starks' life was just an excuse to make it credible - as he said, he wouldn't hesitate to sacrifice Tony to protect the Stone. It wasn't because Strange saw Tony ends up being crucial to victory, though he may well be. It was a way to get the Stone past Thanos' skepticism, which we saw in his "No tricks?" comment.
When Strange hands over the Time Stone, he's already cast the time loop spell on it. In the whole sequence, we never see Strange's left wrist - and in fact, we never see it again in the whole movie. We see full shots of every other character, and we see Strange's face, and torso, and right arm... but not his left hand. Maybe he's got it hidden under the cloak, maybe he's sitting on it, but he's hiding that time loop spell. After Thanos heads to earth, the Stone has a time loop spell active and he doesn't know he's trapped in it. At some point in the future, it will reset time to the moment Strange cast the spell, on Titan before the Snap. The time loop is literally the "endgame" Strange refers to, as it was with Dormammu.
Unfortunately, Thanos is a smart fella and he'll deduce pretty quickly what's going on, just as he deduced how to use the Stone to turn back time in the first place. Once he figures it out, he'll just break the spell and that's that, so there's very little time to act. That's where the time travel in the movie comes in, driven by Ant-Man, who has discovered how to use time vortexes in the Quantum Realm to get where and when you want to go. They can use the Quantum Realm to travel in both space and time, but using them can screw up the timeline and maybe destroy it. (The Time Stone, by contrast, can't move you around in space, but we've seen it can reset time back to a particular point safely.) The Avengers regroup at a base of operations in the future, then go back to various points in time where they know Infinity Stones were available, scenes shown in CAPTAIN AMERICA: THE FIRST AVENGER and THE AVENGERS.
Some theories say they're going to change history this way. That won't happen, it's too Back to the Future for an Avengers movie. They'll need to go undercover (or masquerading as their younger selves) to avoid changing history, because they need to have history happen exactly as it did, up until the time loop. They'll bring along B.A.R.F. tech because reliving and observing memories of a certain point in space-time is the key to traveling there via the Quantum Realm (in quantum mechanics, conscious observation is what causes uncertainty to resolve into a particular reality, a la Schrodinger's Cat). The Infinity Stones are taken out of the timeline and collected for one task, taking down Thanos before he breaks out of the loop or repeats the Snap. The Avengers would be able to use time travel to confront Thanos the instant the time loop resets, and if they win, will be able to return the Stones back to the exact instants in history they borrowed them. That leaves the question of how they're even going to harness the Stones' power, seeing as no one but Thanos was powerful enough to handle more than one, and in the time loop Thanos will still have at least four of the Stones.
And of course the answer to that is the new Gauntlet, designed as much by Shuri as by Stark, and made of nanotech vibranium for its ability to absorb energy and protect the wearer from it. They'll recreate Thanos' journey to collect at least some of the Stones - say, the Space, Mind, Power and Time Stones - and bind them onto the vibranium Gauntlet. (Along the way, it may be the Mind Stone that reconciles Banner's mind with Hulk's mind, and produces a Hulk that integrates both.) I have a hunch the one wielding it against Thanos won't be Thor, but Captain Marvel - you have to be a cosmic badass just to wield one Stone, let alone several. Captain Marvel will be key to retrieving the Power Stone, which has ties to her Kree heritage, and she's just recently reconnected with Nick Fury, who's been busy building a secret underground version of SHIELD to protect Earth. My hunch is that the battle ends not with killing Thanos, but exposing him to the Soul Stone and showing him the devastation his mad journey has caused. He ends up contrite and living out a peaceful solitary existence, just like in the original comics.
Anyway, that's my theory and I'm sticking to it.
Edit: TL;DR: Strange saw the Snap would happen in every possible future. It was even necessary for the future where they win, because it traps Ant-Man in the Quantum Realm and forces him to learn to use time vortexes to escape, leading to a new method of time travel. Strange casts a hidden time loop spell on the Time Stone (like he used on Dormammu) and tricks Thanos into accepting it, by pretending to want to save Tony. At some point it'll reset time to before the Snap. This gives the Avengers one chance to time travel out of the loop to regroup in the future, heist Infinity Stones from the past, and build a new vibranium Gauntlet. They have to do it undercover to avoid changing history, so they can return to the same moment they left. Thanos chases them through the past, but also knows history is too fragile to risk changing it. When the time loop resets, if the Avengers win against Thanos, they can return the Stones to their places in history at the instant they were borrowed. If they lose, the Stones never get returned, and our reality gets completely borked. Once they steal the first Stone, they're literally going double or nothing on the Snap - so they need to be willing to make any sacrifice to win.