I use my phones caseless, and without a screen protector. I’ve done this since the iPhone 6 full time. I’ve had my XS for 3 years. During usage, and even under normal lighting you can’t notice one flaw. However, if you put it under direct lighting, and at the right angles, you will see some light scratches on the front and back. Nothing major.
If you take minor care of your phone, it will stay in great condition. Cases and other protection, for most people, are just a waste of money. Phones with cases can still get ruined.