Notes app not showing on screen

I was surprised to find that you can delete some built-in apps completely from your phone.

Delete built-in Apple apps from your iPhone, iPad, or Apple Watch - Apple Support

Bur if you have just accidentally deleted the app icon from your screen, you can get it back from the App Library.

Organize the Home Screen and App Library on your iPhone - Apple Support

I believe that if an app is in the App Library but not on a screen, you will see an option to add it to a screen in its touch-and-hold menu.

Swipe all the way to the right to get to the App Library. Find the notes app, put your finger on it and drag it back to the home screen.