Man talk about undiscoverable. The face and object recognition in Apple Photos is really pretty good but nearly impossible to find.
Here are some quick tips
To kick it into face recognition most select a photo and then swipe up. Don't choose edit! You will see a list of faces that it finds. Click on the face and choose add name.
If you see there is a mistake in the face then you have to go to the People tab in the Albums and choose select in that view only. There is no other view that it works in. Click the share icon. Which is super weird and y will see an choose "Not this person."