I lost my "notes" app, I do not know what I did, please help
Make sure you haven't misplaced it. Swipe down on the home screen and enter the app name in the search field.
If you're running IOS 14 make sure it isn't in your App Library:
Organize the Home Screen and App Library on your iPhone [iOS14] - https://support.apple.com/HT211345
Is it an app you may have deleted? Read this document:
Delete [and restore] built-in Apple apps on your [iOS] device or Apple Watch - including a list of which ones can be deleted.
Click here --> https://support.apple.com/HT208094
Some apps cannot be deleted but may be missing if disabled with parental controls or ScreenTime as in this topic: https://discussions.apple.com/message/33168221#message33168221
Use parental controls on your child's iPhone, iPad, and iPod touch - https://support.apple.com/HT201304
ScreenTime - https://support.apple.com/HT208982
Settings > Screentime > Content & Privacy Restrictions > iTunes and App Purchases