What Endgame, Loki and the later films say, and what they don't.
The Avengers steal Infinity Stones from 2012, 2013 and 2014. Did that crack open the multiverse? The Loki series gives a surprising answer: the heist itself was 'meant to happen', and the real break came later.
Levels marked "Sourced facts" are checked against the sources listed at the bottom. Levels marked "Fan theory" are interpretation, not confirmed by the films or their makers.
In Avengers: Endgame, Bruce Banner explains that changing the past won't change their present; it creates alternate realities instead. In 2012 New York the Ancient One gives Banner the Time Stone only after he promises the Stones will be returned to their proper points in time. The team gets the Mind and Time Stones in 2012, the Reality Stone in 2013 Asgard, and the Power and Soul Stones in 2014. One thing goes wrong in 2012: Loki escapes with the Tesseract (the Space Stone). At the end, Steve Rogers goes back to return the Stones.
Loki season 1 (2021) picks up that escaped 2012 Loki. The Time Variance Authority (TVA) arrests him for creating a new timeline and resets it. When he blames the Avengers, Judge Renslayer says their time travel was meant to happen, unlike his escape. Later, the variant Sylvie sets off TVA reset charges that create many branches. In the finale she kills He Who Remains, which unleashes a multiverse the TVA can't prune. Wikipedia notes that the season sets up Doctor Strange in the Multiverse of Madness (2022) and Ant-Man and the Wasp: Quantumania (2023), where Kang is revealed as a variant of He Who Remains who was exiled by his other variants.
This part is open questions and fan debate. By the Loki series' own rules, the heist did not break anything; Loki's escape did, and the TVA cleaned that up. The wider multiverse opens because of Sylvie in the Loki finale. Some fans still argue the heist must have left other marks: for example, the 2014 Thanos and his army leave their own time and die in 2023. The films never show what happens to that 2014 timeline afterward, so any answer is a guess. We have not found an official statement that settles it.