I accidentally deleted the texting app. How do I get it back?

Use App Library to search the app you are looking for, 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 app, tap & hold on the app, then drag to the Home screen & drop it.

Just go to the App Store and search iMessage or Messages and the app will appear and you can again download it from there

FaceTime sign in first.

on your contact make sure your mobile is an iPhone.