How does it work with paying with paying off an iphone completely when you first buy it?

Like if you bought an iphone 8 for example for $700 and paid all of that at once, how much would it be per month to have a plan? Like if you did a two year plan paying off the phone and paid $30 a month for example isn't that towards paying off the phone, so what if you already did?

If you buy the phone outright at the start, then all your monthly plan pays for is your calls, messaging (SMS & MMS) and data.

There's no fixed cost for that: it's a competitive market, the cost depends upon which exact allowances you choose each month, and whether you choose to agree to a minimum contractual term.

But if you're buying a $1000 phone and pay $800 up front for it, you effectively get a $200 loan with the repayments plus interest paid back in equal monthly installments over the contract term.

Obviously if you pay off the phone then you officially own it. You can check any of the plans online with the diff carriers to see what you would pay.

If you bought the phone outright/full price then usually people just swap the SIM into the new phone & continue with whatever plan they are on currently.

If you're paying it off over the course of a plan then yes that is paying off the phone over time.