What is replacement iphone?

Hello everyone I just want to know what is replacement Iphone? Because recently I bought a brand new Iphone 14 pro max and after one month using it there is a problem like restarting by its own. I bring my phone to the apple store and they say they will going to repair it because its hardware faulty but the thing my phone is only one month and they change with other phone that i dont know if its new or refurbished. Any one here is the same experienced?