When we last see Tony in Infinity War, he's on the decimated Titan with Doctor Strange, Spider-Man, and the Guardians of the Galaxy (minus Gamora, Groot, and Rocket Raccoon). This is where Thanos leaves them to complete his mission, and we see everyone but Tony and Nebula disappear as a result of the snap. Tony is the first face we see in the Endgame trailer, and he's obviously not doing so well. He breaks our hearts with his final recorded message for Pepper Potts, letting her know that he'll be thinking of her until his last breath. According to Tony, the ship that was (presumably) his way home has no food or water and is quickly running out of oxygen. It's definitely not ideal.
The first thing we can assume is that Tony is on the ship with Nebula, since they were the only two survivors left on Titan after the snap. We even briefly see Nebula in what looks like the Guardians' ship — so it's safe to say the two boarded the Benatar to try to make their way to Earth (though Nebula does have her own ship and normally doesn't play well with others, so who really knows?). The big question is: how are they going to join up with everyone else? Obviously, the journey hasn't gone according to plan, though the lack of food and water aren't as much of a problem for Nebula as they are for Tony. Still, we're expecting there to be a serious rescue mission for the two of them, perhaps from an armor-clad Pepper Potts.
