Nipple sphincters. They will automatically be shut but you can relax them and even open them, but doing so well be hard at first. It will take exercise and practise to open them.
You will however be able to relax them and push and pull things through. It won't hurt, and if an item vibrates it will feel vaguely pleasurable.
You can also close your nipples tightly but it will also take practice like opening them.
When there is nothing in your breasts they will be near flat. If you want to have boobs then you can fill them with liquid or gas. Just remember that helium isn't that floaty. You could also use a solid, mesh or gel implant to get your boob shape back.
Your skin will be very stretchy and form fitting. You could fit like a 3 foot stick in but not a 3 foot tall barrel. Have fun. I'll call if I forgot something.