Time travel is a concept that has been used in many different movies and TV shows. Most sci-fi fans will be used to the way time travel is used in narratives like Back to the Future, where travelling back in time changes the past and creates a new future for the main characters. However, the way time travel is used in the Marvel Cinematic Universe is slightly different -- and time travel is taken to a new level in the 12 Monkeys SyFy show.
As many Marvel fans will know, the MCU describes the multiverse as a series of branching timelines from one core story. The main storyline is the events of phases 1-3 of the MCU, which involves the establishment of heroes like Captain America, Iron Man and Thor, and Nick Fury's creation of the Avengers Initiative. When the remaining Avengers travelled back in time to return the infinity stones to their original places and times in Avengers: Endgame, they created different branches in the timeline along the way.
The way time travel is used in SyFy's 12 Monkeys is more complex. Loosely based on the 1995 movie of the same name, 12 Monkeys shows time as a loop and frequently uses the imagery of a snake eating its own tail to explain how the events in the show play out. The Army of the 12 Monkeys cult exists to find The Witness, who is supposed to create the Red Forest, which is a place where time and death don't exist. For the cult, that's the endgame. To stop time and get to that Red Forest. For the show, meanwhile, there's no beginning or end to the loop, it's just an endless cycle that keeps repeating.
12 Monkeys' explanation of time travel is much more consistent than the theory used in the MCU. The MCU breaks its own time travel rules with Captain America's final scene in Endgame, where Bucky and Sam find out that Steve stayed with Peggy when he returned the Space Stone and grew old with her. According to the time travel rules explained by Bruce Banner earlier in the movie, this should have resulted in Steve living with Peggy in a new branch of the timeline, making it impossible for Sam and Bucky to find him as an old man, unless Steve travelled back to the original timeline -- which isn't shown in the movie.
In a 2019 Reddit AMA, Joe and Anthony Russo, the directors of Endgame, responded to a question about Steve's life with Peggy in the new timeline, stating: "You are correct in that he would have had to have worn the quantum suit, using a pym particle to make the jump [back to the main timeline]. He's not wearing the suit on the bench, because that is not the exact moment to which he returned." While this does clear up some inconsistencies with the use of time travel in Endgame, the choice to exclude this information from the final cut of the movie itself makes the story more confusing for the average viewer.
12 Monkeys does time travel better than the MCU because it establishes rules and sticks to them throughout the entire show. The MCU's failure to explain why Captain America was able to return to the main timeline in Endgame makes the story less coherent and neglects to use the "show, don't tell" method in their filmmaking. In comparison to the time travel in 12 Monkeys, these inconsistencies make the scriptwriting in Endgame come across as unprofessional and lazy. This comparison between 12 Monkeys and the MCU shows how establishing coherent rules and then consistently sticking to them is vital in time travel stories and that even the smallest of inconsistencies can ruin or disprove the entire plot.