1. the money stay on that adress.... so it's not become one on main monero account?
By "main", I presume you mean "primary account". That's the accepted term. XMR sent on a sub-address of an account are part of the account.
i thought they will become one end fund.... all money that received from ALL sub addr isn't all of that will become one? in whole balance of our monero?
All XMR on your sub-addresses are part of your account, and can be spent altogether normally. I don't understand your concern.
do u know any exchanges that still allow monero or trade monero with something like USDT?
Check out eXch:
https://exch.cx/.