Will they just replace a device

Apple will determine the remedy for repair.

Start here.

iPhone Repair - Official Apple Support

If “just bought” means you purchased this iPhone within the past 14 days, then you may exchange it, assuming you bought it at an Apple Store or Authorized Apple Retailer.

That depends on what the issue is and what the diagnosis results are.

If you purchased the iPhone within the past 14 days then you can just return it and get another one.

Axel F.