Camera Roll

Some general info on the Camera app, and this poorly-documented part of it:

Take and edit photos on your iPhone, iPad, and iPod touch - Apple Support

About the Camera features on your iPhone - Apple Support

Use camera modes on your iPhone, iPad, and iPod touch - Apple Support

And here’s where you tap to the access the camera roll, within the camera app:

in the photos app.

In the Camera app. There’ll be a square thumbnail shown and tap-able, if there are images in the camera roll.