accidently deleted reminders app on iPhone

It sounds as if you deleted the app from the Home screen but not from the phone.

See here:

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

Hi there,

It sounds like you removed the reminders app from your Home Screen but not from your phone. Not to worry, it is just in your App Library. To get it back to your Home Screen simply swipe your Home Screen to the left until you are on the last page, there should be a search bar at the top, so go ahead and search for the reminders app there. When it pops up press and hold on it until it jumps up a little then you can drag it to the left side of the screen to bring it onto your Home Screen. You can keep holding on to it and dragging it until you get it to where you want it.