Change photos into videos or slide shows

You should see this:

Make and play slideshows on iPhone - Apple Support (EG)

It says this:

1. Go to the Photos app  on your iPhone.
2. Swipe down to see the photo grid, then tap Select.
3. Tap each photo you want to include in the slideshow, then tap .
4. Tap Play as Slideshow.

Note: Your slideshow is automatically saved as a memory. To view it later, scroll down to the Memories section of the Photos app.