Science fiction often gets dangerously close to being philosophy. When you make a decision does the world in which you made the other choice exist in any real sense? Is "now" the trunk of a tree of myriad branching possibilities? If not, then why do anything - the whole history of the universe is a predetermined train ride with only one set of rails. If not, how do we get to the other universes?
This cartoon from 2018 explains it very nicely: https://abstrusegoose.com/592
My favourite treatment (until recently) is in one of the Alastair Reynolds books. A scientist working on time travel is injured in a laboratory accident. His colleague isn't just vaporised, but his entire history is erased. He never existed, except in the scientist's memories. The scientist is driven mad by living in a universe where nobody remembers his friend and his life left no physical evidence. He eventually finds suitable grandparents who _could_ have produced him.
On Boxing Day I got a Christmas card from a closely related alternative reality!
It was delivered by hand from the family across the road and it was addressed to "Richard, Marie, Charlie and Evie". I'm single and live alone, but in the reality that the card came from I'm married and have children. The design of the card itself gives no clue about the differences between our universes, and the printing company exists in both. I'd say that the fact it was delivered on Boxing Day indicates that when the Catholic church invented leap years to stop Easter marching off towards midsummer they came up with a slightly different algorithm - be honest, you never really understood whether 2000 was supposed to be a leap year or not.
Hot Marie used to often come to my band's shows and I've been to her house a couple of times. In this universe she never seemed interested in me, but given the chance I would definitely have married her, and I would have taken every opportunity to try to impregnate her.
The most inexplicable thing about the other universe is that my oldest son is called Charles. I would have expected it would be John, like my father and his first son. Marie probably chose it, or I already had a nephew called John.
Or maybe the chap two doors down with the girlfriend and two children is also called Richard.
Richard "I'm in the wrong universe. Let me get off." B