Jesus was likely an apocalyptic preacher who was crucified by the Romans but developed a following in the course of his ministry. Shortly after his death (and maybe just before), his followers saw him as a man who had the “divine” about him, like a messenger from God. Eventually, his followers started treating him like he was still a man but who was adopted by God as his son. Then eventually (maybe early 2nd century?) you eventually had this notion of Jesus as actually God. They started with a man and kept heaping glory on him until they made him God.
So over the decades, you have all these contradictory texts about Jesus’ divine versus human nature. When you get to the 3rd and 4th centuries, eventually church leaders wanted to create one harmonized view of who Jesus was. But the the problem is your sacred texts all describe him in contradictory and mutually exclusive terms.
So that did they do? They came up with the doctrine of the trinity to try and address all these contradictory views into one doctrine. But the trinity fundamentally does not make sense because it tries to take these contradictory views and mash them into a whole.
A good book if you want to know more is Bart Ehrman’s How Jesus Became God.