iPhone camera reproduces 100's of photos from my library that I have not taken with my iPhone

All of the photos on your phone are in one place, in the Photos app. The square in the lower left corner of the camera just opens the Photos app. But you can sort it so you can see the most recent photos. And there’s even a Recents album.

They are different apps, but they sync through iCloud if you have iCloud enabled on both the Mac and the iPhone.