How do you write alternative endings to a novel, comics or movies? Video games do this, because they allow players to take decisions, but movies, comics and novels don't really allow for that, so taking into account that you can't take decision, how do you show alternatives endings and alternative timelines? Can you have a story that branches out at point B, which is in the middle of the story and diverges to 2 different endings, how would you achieve this in a novel, comics or movie?
If you are writing a series of stories, novels, TV episodes, or something about the same characters, you can set the different stories in different episodes.
The more episodes an adventure TV series lasts, and the more dangers the protagonists face, the lower the odds are that they will survive until the end of the series, and the less plausible their survival is.
Suppose that in the average episode, the protagonists have a fifty percent chance of surviving and a fifty percent chance of being killed. Their chances of surviving ten episodes would one in 1,024 or 0.000976562, their chances of surviving thirty episodes would be one in 1,048,576 or 0.000000953, their chances of surviving thirty episodes would be one in 1,073,741,824, and so on.
And a lot of adventure TV series lasted for a lot more than thirty episodes and the protagonist's odds of survival often seemed a lot less than 50/50 in each episode.
So I developed the idea of imagining that in a highly episodic and non-serialized TV series, the tens or hundreds of episodes each happen in a separate alternate universe of their own (except for the few episodes which are clearly sequels to other episodes). You can imagine that the creators of the series searched though thousands and millions of alternate universes to find experiences of the protagonists having experiences suitable for making episodes about.
I developed this concept about Star Trek: The Original Series, but it can be applied to other science fiction, adventure, or even comedy television series and long-running series in other media.
Suppose that a protagonist of a comic book or comic strip is a child and they don't grow older over the years and decades it lasts. If it is a serialized strip and all the stories have to happen one after another, then the kid should age by the total fictional duration of the various fictional events, and if they don't, that is a fantasy or science aspect of a series which otherwise might be totally realistic.
But if the various stories have no connection to each other, then it can be imagined that all the stories happen at roughly the same time in different alternate universes, and thus someone can accept the child not seeming to age over years and decades. So the writer of a comic book or comic strip can make it clear that the different stories all happen in different alternate universes if they wish.
Thus, it is perfectly possible for a writer of a series of stories to write a scene where a character tries the wrong combination to open a safe containing something important and gets electrocuted or releases poison gas and dies. And follow that with a scene in an another alternate universe where the character tries a different combination and it is also wrong and they also die. And a few more such scenes. And then follow with a statement that the character tries the wrong combination and dies in 999,999 out of a million alternate universes. And then have a scene in one of the alternate universes where they try the correct combination and live, and set the rest of the story in that alternate universe.
Or maybe set the rest of the story in an alternate universe where the character wisely declines to risk their life trying random combinations and backs off and tries another course of action entirely.
In a science fiction or fantasy series you can have the protagonists visit or have contact with alternate universes and find out that in those alternate universes, things went differently than in their universe and they were killed during experiences they survived in their own universe.
Watching Duck Tales (2017-2021) I noted that the protagonists constantly survived taking foolish risks which should have killed them, and the odds that they would survive to the end of the series would be extremely small. I have an urge to create a sequel to Duck Tales where someone who can travel between alternate universes takes the ducks to many alternate universes where some or all of them were killed during adventures they survived in this universe. The traveler shows each of the ducks examples of times when their actions got their family members killed, and shows that alternate universes where they all survived are outnumbered thousands to one by alternate universes where some or all of them died.
Using alternate universes can be a good way to show which courses of action are more likely to result in success.
Or it can be used to show that different courses of action do not have guaranteed success or failure, but merely highly or lower probabilities of success.
One way to show that courses of action have different probabilities of success and not guaranteed success is to write alternate universe versions of a battle scene where the protagonist does the same thing in every version but in some alternate universes is killed or wounded by an enemy arrow, bullet, or ray gun, while in others he survives unharmed.
"Video games do this, because they allow players to take decisions, but movies, comics and novels don't really allow for that."
You kinda answered your own question. The difference being the choices you make. In every story characters make decisions all the time, they choose to spare or kill somebody for example what can drastically alter the course of the story.
So what you need to do is look for these choices and then ask yourself, what would happen if this character made a different choice at this moment? And go on from there.
An example would be, what if Luke accepted his fathers offer in Empire Strikes Back? How would Return of the Jedi be then with Luke having trained under Vader and they did a planned attack against the Emperor?
Or when Frodo stops Sam from killing Gollum, what would happen then without Gollum/Sméagol being able to guide them in to Mordor?