Assuming no one else responded to this yet. Your full statement balance will be the full cost of the phone + other purchases. If you owe $1000 on a iPhone, and used the Apple Card for another $400 in groceries, your statement balance would be $1400, and it would report as such. With that said, your minimum monthly payments would be adjusted higher to accommodate the installments. For example, if your installment payment is supposed to be $41.66/mo, and your Apple Card's minimum payment is $25, then your new minimum payment will be $66.66 for that month. Your minimum payment will continue to be at least $41.66/mo, even if the Apple Card is completely paid off except for the iPhone Installments balance, until your iPhone is also paid off.
To do iPhone Installments, you must have the available credit limit as if you're paying for the iPhone outright (i.e. you cannot do iPhone Installments on a $1000 iPhone if your Apple Card is only a $500 card, or if you have a $2000 Apple Card but you already owe $1200 on it.). Since this will count as credit utilization and not a separate loan, take care that you try not to use more than 30% of your Apple Card's total balance (and of course, across all credit cards together), or it could negatively impact your credit score temporarily.
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See my post above.