With dozens of characters to choose from, Christopher Markus told us that they just had a wall of characters they were looking at and would seemingly randomly put any two of them together and consider what sort of story they could tell or what sort of humorous interaction might occur.
Avengers: Infinity War, is that same concept but now multiplied by a factor of 10.
The entire premise of the Marvel Cinematic Universe at the begging was essentially, let’s create this handful of characters and then see what happens when you put them all in the same room together.