Where can I find the photo mosaic feature?

Jeffrey Shaw wrote: …individual pics flying in — starting large and shrinking into place —to form a mosaic that was gradually revealed as one of your photos,

That sounds very cool! I can imagine it, but I don't remember it in Photos, at least. Photos doesn't have a tool to make a collage, much less a flying one. If it's a collage you want, there are apps for that at the App Store. Because I already have this very versatile app, I used GraphicConverter ($40) to make this:

that I liked better than the special apps I tried.

Probably the best way to do a flying collage would be to use the Keynote app that comes with the Mac. It has great animation features, and you get lots of control. Keynote is pretty spectacular!

There's a slideshow theme in Projects ➙ Slideshow that may be what you're looking for. It' called "Sliding Panels". If that's not what you want check out the other themes.

If you can't find what you want you could create the slideshow in Keynote. It has some amazing features for slideshows.