This is divided into sections to make my (and your) life a little easier. You can skip around to whatever intrigues you most.
The holiday
2023 years ago little baby Jesus was born, and that was the start of everything. The first recorded Christmas was a celebration of Christ's birth in Rome on the year 336, and I'm not sure exactly when the word "Christmas" started being used but as for the etymology of it..
Santa Claus
While reading the previous section you might've thought, "Okay, when does Santa come in?"
And I am here to quench your Santa Claus thirst. I decided to make it a seperate section because it's kind of weird and convoluted, and I wanted to skip around in the timeline as least as possible.
The concept of a Christmas gift-giver is something that independently happened in many different places, and Santa Claus is sort of an amalgamation of all of those guys. But the earliest inspiration to Santa Claus is, of course, Saint Nicholas. Or more specifically St. Nicholas of Myra (sometimes called St. Nicholas the Wonderworker) as just "St. Nicholas" has become a sort of mythical figure, only based on the real St. Nicholas of Myra and more akin to Santa Claus. But anyway. St. Nicholas was an early Christian bishop that resided in what would now be Turkey, and during the time of the Roman empire. Not that much is known about real life St. Nicholas, but the most popular (and earliest told) story from him was that he rescued three girls from being forced into prostitution by secretly leaving them bags of gold (put in through the window in their house) so their father could pay a dowry for each of them. He was known for giving to the poor and for secretly leaving gifts to people, which would soon influence Santa Claus. The two other Christmas gift-givers that influenced Santa the most were Father Christmas and Sinterklaas (Sinterklaas mostly just gave Santa Claus his name, so I won't talk in-depth about him). Father Christmas was a personification of Christmas that had it's origins in pre-Christian winter celebrations (such as the aforementioned Yule and winter solstice) and he also was associated with and resembled Odin, the leader of the Wild Hunt (a folklore motif that's basically a chase by a mythological figure escorted by a spirit/ghost? Idk. It's so complicated) during the time of Yule.
The Christmas Tree
This will be shorter, I promise. During the winter time, some pagans would put evergreen trees in their homes during Saturnalia to remind them of the summertime, and possibly also because evergreen plants are considered sacred in some pagan belief systems. With Christmas taking over pagan holidays, people started to decorate their evergreen trees with Christian elements such as apples (to represent the garden of Eden), candles to illuminate the tree, white wafers (to represent the Eucharist and redemption), and others. The modern Christmas tree took shape around the rennaisance, with the apples being switched to ornaments. With electricity, the candles were changed to lights, and we started decorating them with tinsel and other extra bits and bobs - I'm not completely sure about when the practice of the Christmas topper was introduced though, but it also has Christian roots with the star topper representing the Star of Bethlehem that led the three wise men to baby Jesus, and the sometimes used angel topper representing Gabriel.
Other stuff
Here is some old Christmas advertisements from around the 1940s I found