Is there a iPhone mail app that I can download
No you
iPhone 13 Pro, iOS 15
Question marked as
There is a Mail app on your iPhone.
If it is not on your 'Dock" or Home screen, try to use App Library to search Mail app, see this Apple article:
Organize the Home Screen and App Library on your iPhone
Organize the Home Screen and App Library on your iPhone - Apple Support
If you have find the Mail app, tap & hold on the app, then drag to the Home screen & drop it.