Why is there unknown hidden box on iPhone 13?

That is the Hidden Apps folder, where you can hide select apps from others which requires Face ID to enter. The apps inside the folder show up as hidden, otherwise that would defeat the purpose of the folder.

Learn more from Apple: Lock or hide an app on iPhone - Apple Support

The App Library is an integrated feature of the iPhone Operating System (iOS).

The Hidden folder 📁 is dedicated for apps you wish to isolate from accessing and accidental deletion. This special folder is secured by Face ID. 🔐

All the best! 👋🏼😉