You bought a $649+ iPhone 7 last year.
After a year, your monthly payments will have paid for half of it.
You've paid nothing that would contribute Towards a new phone. You still owe $325 to finish paying off your iPhone 7.
Also, if you want to keep your iPhone 7 when you upgrade to the next iPhone, you can. Simply pay off the $325+ you still owe for it. That closes out your old contract, but you get to keep the phone (and sell it yourself).
For most people, trading the old iPhone back into Apple is easier, because Apple then pays off the $325+ they still owe for it, which closes out the contract.