The invisible world of parallel realities fact or fiction

There are many stories about ending up in a parallel world, both in a dream and in reality. That world where all the usual things look similar, but still different; where a person's fate could have turned out differently, or even historical events took a different path. What is our reality where we live? Are we the only one group of inhabitants on the Earth or not? Do parallel worlds exist? If yes, than how to get there and how did they appear?
What is this - fiction, a play of imagination, or an unknown reality? I believe that among such stories, fantasy and fiction are certainly present. However, there are definitely true stories and testimonies of people who have encountered the unknown... To understand this and explain what's what, I'll start my story about parallel realities. I am interested, first of all, in the "mechanism" of their formation itself. And I will try to tell readers about it in the most accessible and understandable form.