(**Warning of major spoilers for the original theatrical release of Avengers: Endgame ; and various other media including Back To The Future ). Includes link to a Disney Plus article by Comicon’s own Erik Amaya; and a link to a Tech Insider youtube video from 2018 explaining multiverse theory featuring astronomer Stuart Clark.
Avengers: Endgame is the 22nd movie in Marvel Studios ongoing multi-movie franchise. It acts as a stop point for a lot of elements from the previous movies. Yet it is also a bridge to the next series of movies, and some shows, in this ongoing franchise. The plot of this movie continues from the events of Avengers: Infinity War (2018). Said plot has aspects of both dystopian future films and revenge movies, though these elements do not encompass the majority of the movie’s plot. Instead, we get a time travel story featuring superheroes.
However, there are different types of time travel stories. There are the ones like the movie Back To The Future where after a trip to the past, the present that one returns to, too, is altered. Another type is the one Dragon Ball Z (1989-1996) uses that suggests going to the past will not change the travelers’ present due to multiverse theory. This type suggests alternate timelines/universes are created by any attempt at changing the past. A final example is the plot-centric type that has the rules change with the needs of the narrative (Doctor Who [1963-1989, 1996, 2005-Present]). In this case while the rules of time travel are explained in story, they may give way to the needs of the plot.
Avengers: Endgame technically uses all three types. It uses the first type when Steve Rogers (apparently) returns to the ‘present’ at the movie’s end. Presumably the Steve Rogers that goes back to return the stones stops his trip in the 1970s. Thereby allowing the events of Agent Carter (2015-2016) to occur, and solving the mystery of who Peggy Carter mentions marrying in Captain America: Winter Soldier (2014). We can thus assume that he and Peggy create a fake identity for him, and her later dementia prevents people from believing the truth. However, this is never shown in the movie and therefore is debatable.
Less debatable is how the Avengers: Endgame uses the second type. In other words the Avengers teams do create alternate timelines/universes. To better understand this concept one should consider how the Avengers take what look like tunnels when traveling to the past. Each of these tunnels heads to a different point in time in a similar way to a branch on a tree heads to different areas of the sky. Also the events of 2012 go differently than what the main MCU Steve Rogers & Tony Stark remember (mainly Loki escaping with the tesseract). Therefore we can assume that whether stealing or returning the stones, the Avengers actions are making alternate timelines/universes.
The last of the types co-exists with the second type of time travel. This is due to way in which the 2014 Thanos learns about the time heist and comes to the main Avengers present. Presumably Iron Man kills Thanos and his army as his only action since he is only able to use the infinity gauntlet once before dying. With these two actions we get some slight additions to the rules of time travel. The first addition is the confirmation of alternate timelines/universes existing. While the second addition is that time travel can apparently only send one to an alternate past. (Which does make one wonder if old Steve Rogers is the same one they originally sent back.)
In conclusion this movie is a compilation time travel story that leans more toward Dragon Ball Z and Doctor Who in style.