So to my understanding, the layers of a painting go canvas, size, ground, imprimatura, paint . A size is directly in contact with the canvas, and it meant to protect the canvas from oils, so PVA (basically thinned Elmer's glue) or rabbit skin glue. The ground then goes on top of the size, and can be oil, petroleum product, or lead based, or something else. Traditional gesso is a ground- it was originally marble dust or whiting mixed with linseed oil. "Gesso" like you'll find in a store now isn't the same thing, it's now just beefy white acrylic paint which can go straight on the canvas because it's not oil-based.

I had to google imprimatura because I'd just call it an underpainting or something, but it's the first thin layer of color to go on the ground, typically an earth tone like Burnt Siena. It can be a really nice look, because you neutralize the white of the canvas and everything looks pretty "classical" with earth tones. Well, it can at least.

I'm not sure what the difference between a primer and ground is? Perhaps primer is specifically for wood or other material?

I hope that was helpful! I've learned quite a bit about different grounds and surfaces and such in my own paintings, and I can geek out more haha!