If you do something major, a new universe with an alternate timeline is created, which branches off from the universe the event happened in. So if something happened, a new universe is created, and there is also a universe were it didn’t happen. However, you wouldn’t be moved to a new universe because the timeline before the major event was already created and there, even before the major event.
If you make a decision there is no going back in time and also no alternate reality appears. What’s done is done and you gotta live with it. Unless of course the decision killed you.
Though there are other realities that all branch down from one major reality and each reality is different with different creatures, people, weapons, civilizations, biomes, buildings, cultures, religions, etc.
because if new universes were created based off of major choices then there would be several ultra powerful wandas roaming about with the power to travel the multiverse all trying to go to the same place at the same time
There are only a few universes were the events of WandaVision would take place, and of those universes, not all of those Wanda’s would do what the Wanda from the “main” universe did.
Also this topic is about how marvel is getting mid so let’s not argue.