“Stephen King’s It is a complex story in which a lot of things happen, and at times things get a bit confusing. But don’t worry about getting lost in the story — we’re here to explain everything you need to understand the full story of the movies made from the classic novel. Possible spoilers ahead.
“2017’s It didn’t get into the backstory of its ancient antagonist outside of implying that it’s been around for a long time, but thankfully, King’s original novel goes into much more detail.
“It, the shapeshifting being that often manifests itself as a creepy clown named Pennywise, has actually been around longer than mankind itself — It’s an ancient cosmic being that’s billions of years old that originated in another dimension outside of our universe known as the Macroverse.” – YT