Marvel Studios are unique in cinema today — perhaps even in the whole of modern Hollywood history — in that they are a studio dedicated to making only one "type" of movies (superhero movies — for all the kind of different genres they move in and out of). Not only that, but every single one of the studio's movies is set in the same world, along the same timeline, connected to the rest of the company's output by crossing over characters, devices, events and McGuffins, so that the entire creation feels as much like long-running serialized storytelling as it does a movie slate.
How Marvel keeps all this world coherent and cohesive is a large part of the studio's success, and of what keeps audiences coming back, consistently, to enjoy it, at a time when similar production companies — Warner Bros.' DC the most evident example — struggle to achieve the same sense of excitement amongst audiences. And sure — the obvious answer is that Marvel relies on a large cast of well-executed classic characters, cross-pollinating their stories, ensuring that each new movie feels like a sequel to the previous one, even if it's about a new protagonist: superheroes appear in each other's movies, larger storylines (Infinity Stones, anyone?) drive the overarching narrative forwards to a meeting point.
But there's more to the thick intertwined fabric of the MCU than cameos and McGuffins. There's a whole web of ingredients, inside jokes, and recurring business that make the films feel of the same brand — whether the film is Guardians of the Galaxy's space opera or Ant-Man's comedy caper.
Consider, if you will, the following a list of minor (but necessary) ingredients to a Marvel Cinematic Universe as it dawns on its transformative Avengers: Infinity War event:
32 mid- or post-credits scenes.
18 Stan Lee cameos, one for every movie released thus far.
9 movies in which Captain America's shield makes at least an appearance, making up for one in every two MCU films. Bucky Barnes — who becomes Cap in the comic books — uses the shield 3 times.
8 severed hands/arms, one of which (Aldrich Killian's) grows back, one of which was never severed in the first place (Thor's), one of which is the metal replacement of a previously-severed human arm (Bucky's).
7 uses of the number twelve. Tony Stark went "12 for 12" with the Maxim cover models; Steve Rogers is used to performing for crowds of 12-year-olds as Captain America; Pepper Potts' "12 percent of the credit" for Stark Tower; Tony Stark's quip that Maya Hansen may have a 12-year-old waiting in her car; Rocket Raccoon's quip that 12 percent of a plan isn't a plan but a concept; Quicksilver's being born 12 minutes before his twin, Scarlet Witch; Sam Wilson's Falcon wings are hidden behind a 12-inch steel wall.
5 instances of Tony Stark's left arm being injured, numb, cut or painful, and judging from various Infinity War promotional materials, which show both Tony holding an injured left arm and donning an Iron Man suit with a bionic arm, it's reasonable to wonder if one of old Tony's arms is about to be added to the previous list.
5 excellent movie-spanning dialogue call-backs (your clue for each: "I can do this all day," "Adopted," "Point Break," "the price of freedom," "I'm with you 'til the end of the line").
3 Community cameos — see if you can spot them yourself: Donald Glover, Danny Pudi, Jim Rash.
3 instances of Tony Stark not liking being handed things.
The recipe changes (we're told Kevin Feige has a thing for electrocution now — one more thing we can hold against Star Wars: Return of the Jedi). But if anyone out there (cough Warner Brothers cough DC cough) wants to experiment with superhero world-building: it's not all action sequences and hurried team-ups. The devil (and, you know, the 20+ movie gazillion-dollar grossing franchise) is in the details.