How have species evolved from a single cell organism?

I’m not asking you to tell me how it went from single-cellular to multi-cellular. I’m asking like how did species get all these complex organs and bodily functions? Sure, the heart pumps blood and the brain gives us intelligence and controls the rest of the body. Who comes up with this stuff?
Is it proof that God exists? Or proof that adaptation is a lot more complex than we think?

I’m agnostic and I believe in evolution, yet I’m still questioning it.