Originally Santa Claus was just a cartoon used to sell Coca Cola products. But as he became more popular, his lore had to be fleshed out. First we had "Twas the Night Before Christmas" which gave us a lot, including his belly being like a bowl full of jelly. Then we as "Santa Claus is Coming to Town" which added things like the "he sees you when you're sleeping, knows when your awake". However, there are still lot of questions about how Santa operates. And over the years, movies have gone a long way to fill some of those gaps.
Here ae five examples of what I mean.
"How Does Santa Deliver to the Whole World in One Night", Santa Claus:The Movie
This movie is basically an "origin" movie so it makes sense that it would explain some things. The one part I like is when Burgess Meredith's character, "Ancient Elf" I guess, explains that the time continuum breaks down when Santa is in his sleigh and "time moves with you". Meaning that Santa is moving super fast from our point of view. This is something that stuck, it gets mentioned in The Santa Clause and even the much more recent "The Santa Chronicles" used this.