photos app slideshow

When I create a slideshow project in Photos 8 on Ventura and then export it as a video by clicking the Export button, the saved video will have the filename extension .mp4 .

How to create the slideshow project is described in the first link posted by Ralf.

Please have a look at these descriptions

Create slideshows in Photos on Mac - Apple Tugi (EE)

Export photos, videos, slideshows, and memories in Photos on Mac - Apple Tugi (EE)

Hope it will help

Ralf