Today I looked at a giant picture of my own eye and it was scary

I went to the optometrist's office today to get a floater in my eye checked out. During the process, I had to look at a close-up picture of the inside of my own eye. 

I can never understand how surgeons and other people who work with flesh and blood do what they do. And it's even weirder when I'm the one looking at my own internal organs. Like, the three places you don't want to see close up are the inside of your ear, the inside of your nose, and the back of your eye. They all make one feel especially strange when looked upon. 

This is a peculiar feeling, and that's about all I have to say about it. 

On to one of the questions I've always had. I've done finicky work with small parts (instrument repair) and at least once per overhaul I would make a rather serious mistake. Breaking a spring, adding a dent, losing a screw. How do surgeons deal with mistakes like these? Do they just not happen because of experience? Or do surgeons just have hands that are adept enough to fix all of their mistakes before they spiral out of control? 

I could google it but I'm not certain I'll find an answer. 

Also, how do surgeons practice? I would think that operating on a live patient would be too dangerous to let a beginning surgeon do. So, how do they gain experience? Cadavers? How many cadavers does it take to make a competent surgeon?

Anyways, these are the questions that I want answered. Maybe you can find out the solution for me.