Found a hairline scratch on my iPhone 13 max

You are probably fine, it sounds like a surface level scratch. If it is a crack, you may want to get the screen replaced however.

If is only visible when the display is off, absolutely sounds like a scratch.

It is unfortunate that this has happened, but it may have even just been some pocket lint etc. In future, as long as you can still see your screen through a smashed screen protector, you shouldn’t remove it until you have a replacement on hand to replace it with. I have a friend who’s parent removed it before a new one arrived, and smashed the actual screen in the meantime.

Scratches are a near-inevitable part of life when any device is actively used, whether we’re discussing aluminum, steel, or glass.

As an example of life with scratches, beach sand is routinely quartz, and quartz is quite hard (mohs 7), and hard enough to scratch most other substances. (Glass is ~5 to ~6.)

The fancy word for what happens here is “patina”, as in “a well-used device will acquire a patina”.