Worried about data transfer

So gonna get the iPhone 17 tomorrow upgrading from the 12 Mini 128GB which is gonna be mega upgrade. But just wondering how the data transfer works. Does it copy data and then add it to the new phone or does it copy and completely wipe the old one? I also have 3 note pad docs with my passwords on them behind Face ID lock so just worried that they won’t transfer over. Idk if anyone knows the process and can re assure me would be great. Thanks