Wrong, and wrong. Keep pace with the literature. Differences can manifest as early as embryogenesis and have far reaching outcomes, e.g. there have been reports of different covid severity outcomes within a twin pair.

Hell, you do know that the cells in your own body don't all have the same genome, right?

As I said, it's irrelevant since assignment is mostly random at birth. The law of large numbers takes care of trivial differences since those differences are largely randomized.