It's super impressive to me, and getting this kind of fidelity is honestly brand new. You can include an image in an AI image generator to serve as an outline. In this case a blank image with bold text. It'll fill in the prompt using the reference as best it can (in this case, a bunch of kittens/puppies).

Prior to a few weeks ago, I'd only ever seen this using a posed figure--basically a stick figure--to generate the prompt's subject around.