How do I turn off summarize photos on iPhone SE?

Which album or collection are you viewing?

The months or years will always be summarized, but when viewing "All Photos" or one of your albums tap the button with the grid at the bottom right of the screen to switch between the summarized view and "All".

Also check the menu with the two arrows in the lower left corner to see, if a filter is set.

Here's the Summary button at the bottom of an Album view that léonie mentioned:

I don't think I've ever wanted or used the "Summary" choice…

You're welcome 😊