I accidentally deleted photo app. How can I download it again? Thx

Use App Library to search the Photo app, see this Apple article:

Organize the Home Screen and App Library on your iPhone

https://support.apple.com/en-us/HT211345           

Hello saki98, This Apple Support page my help. When you delete a built in App other things can happen too. There are options to simply remove an app and retrieve it without actually deleting it.

https://support.apple.com/en-us/HT208094