Unlock phone

I have 3 phone lines with Tmobile. Can I get a new iphone 18 at apple through tmobile plan but finance it with Apple. I want to give the iphone 18 to my family in Vietnam. My concern is 1: is the 18 phone an unlock phone? 2: does it have any effect on the phone number that be used to trade in? 3: my plan is go5plus, will it get a credit of $830? the trade in phone is iphone 15 promax Thank you in advance!