Why is Christianity a thing if Jesus was Jewish?

Hello, I am currently interested in learning about religion as I am atheist but I still find it interesting. My question is, what is the main purpose of Christianity if it is a religion about Jesus if Jesus was a Jewish prophet. And is God himself not about Christianity, but instead he is just an all powerful symbol that wants to rid the world of sin, and him coming to Earth as Jesus was to spread the message and rid the world of sin, but then Jesus's message becoming its own religion?

Also, as a side question, before Jesus fasted for 40 days in the desert, how did God talk to him in front of all those people if Jesus is God?