A very good question - in the very first version of Photos, long, long ago, it has not been possible to see the info when working on a project in Photos. I wrote this user tip on how to see the info while working on a book: How to Access the Info Panel While Workin… - Apple Community
It is now possible to open the info for the the photos in the film strip while working on a slideshow. But we are still not seeing the Info while playing the slideshow. we would have to copy the info to Title Slides.
I am using this work-around to see the location and date while playing my slideshow: Script: A Slideshow With Titles and Locations And Dates in Photos