Thanks to all. Great to find such an active community. As I said, I’ll have many more questions, I’m sure.
As to this request, the little triangles worked after I toggled with the Tab key; before that, all those arrows did was shift the image either right or left, revealing nothing. But not, thanks to you all, I think I understand how that works. (And I did check the online manual, as well as YouTube, before posting here, though perhaps not carefully enough. I mention this because, as an emeritus Google Photos Product Expert, I often wished that users would check basic instructions first.)