Where Have the Guardians of the Galaxy Been?
Guardians of the Galaxy Vol. 2, directed by James Gunn, ended with the Guardians watching fireworks in outer-space with the Ravagers, mourning the loss of Yondu (played by Michael Rooker), who gave his life so Quill would live after they defeated Ego (played by Kurt Russell). We were given no indication of where they were going to go next or the contents of their plan.
A considerable amount of time has passed since Guardians of the Galaxy Vol. 2 and when they appeared in Infinity War, it seemed as though they had come out of nowhere (not to be confused with Knowhere). Maybe we should assume that they were just cruising through space, doing a bit of good a little bit of bad, but it wouldn't make sense. Nebula left at the end of GotG2 because she wanted to kill Thanos. We see how that turned out in Infinity War but it's never explained what Quill (played by Chris Pratt) and the rest of the gang had been doing since we last saw them. You'd think that they'd also be looking for a way to prepare themselves against Thanos, but that clearly wasn't the case.
Why Wouldn't Hawkeye and Ant-Man Show Up?
There were a wide variety of theories revolving around the absence of Hawkeye and Ant-Man from marketing for Avengers: Infinity War. These theories ranged from secret missions to mourning periods. Anthony Russo then stated that all would be explained when we went to see the film. He was right, of course, and the answer to Hawkeye and Ant-Man's absence is that they took deals to avoid house arrest after the events of Captain America: Civil War.
That'd be an acceptable excuse if we were talking about the Avengers going up against a group of HYDRA agents, but the world-ending threat of the Mad Titan, Thanos, is just a little more serious than a broken secret organization. It makes no sense that Hawkeye and Ant-Man wouldn't breach their agreements for the biggest fight the universe has ever seen. Luckily, there's clearly more to the story. Hawkeye and Ant-Man will be appearing in Avengers 4, and the former will have a very different look. Hopefully we'll get a more solid explanation for their absences, eventually.
Where does Thanos End Up?
After having Thor's new axe fixed into his chest, Thanos snaps his fingers, eliminates half the universe's population and then quickly falls back into a portal, appearing in a rural place, seemingly ready to rest now that his goal has been achieved. Where exactly is that bright, beautiful place and why did Thanos go there? Fans have made a few guesses, some of them are based on the fact that the film took inspiration from the 1991 comic book crossover event, The Infinity Gauntlet. That's where we might find the answer to this question. When Adam Warlock travels into the future at the end of Infinity Gauntlet #6 (written by Jim Starlin, art by Ron Lim, Josef Rubinstein, Max Scheele and Evelyn Stein), he finds Thanos living out a peaceful life as a farmer on an unknown planet where he contemplates his actions and his failures. We can't be sure if this is the same fate Infinity War had for Thanos, but it's entirely possible.
The paradise Thanos smiles at in the final shot of the film may also be a pocket-dimension within the Soul Stone which, in the comics, houses souls and creates a paradise for each and every one of them. This could very well be the place to which Thanos retreated after his victory, in order to rest and recuperate. He'll definitely be seeing him again in Avengers 4, either returning from a planet somewhere across the universe or out from the pocket dimension of the Soul Stone (assuming the film's Soul Stone actually is like its comic book counterpart).
Directed by Joe and Anthony Russo, Avengers: Infinity War stars Robert Downey Jr. as Iron Man, Chris Evans as Captain America, Chris Hemsworth as Thor, Mark Ruffalo as the Hulk, Jeremy Renner as Hawkeye, Scarlett Johansson as Black Widow, Anthony Mackie as the Falcon, Paul Rudd as Ant-Man, Elizabeth Olsen as Scarlet Witch, Tom Holland as Spider-Man, Benedict Cumberbatch as Doctor Strange, Chadwick Boseman as Black Panther, Chris Pratt as Star Lord, Zoe Saldana as Gamora, Dave Bautista as Drax, Bradley Cooper as Rocket, Vin Diesel as Groot, Tom Hiddleston as Loki, and Josh Brolin as Thanos. In theaters now.