No need to apologize, Chris. The confusion is very understandable.
It's not so much that the meaning has expanded, but that common usage and fine-dining usage are different.
We all pretty much grew up where "garnish" meant tossing a hunk of curly parsley on top of the plate. And most cookbooks and other sauces use the word that way---garnish by tossing bits and pieces of decorative this and that on the plate. This often led to abuses in which inedible products were often used, just because they were pretty.
In the broader sense, garnish is anything edible that compliments the main dish. This could be the common bits and pieces of this and that. Or it could be what are commonly called side dishes. Broaden that out, in your mind, and you'll see way: "Side" actually means "aside," something that takes second place to the actual main event. At a circus we have a side-show that really typifies this. What happens at the side-show may be interesting. But what we're there for is the big top.
So, let's look at my example above. The menu item would be grilled pork chops. If I merely put them on a plate, I would satisfy the menu description. But instead, I garnish them with a dollop of mashed sweet potatoes. The dish is still grilled pork chops, because that's the main element.
Where it gets difficult is with something like the typical American at-home presentation of a plate divided in three. Kuan's starch and veggie accompanying the main protein. We tend to see that as three complementary dishes, and not think of the sides as garnish. But let's look at this: The main is a pan-fried chicken palliard. For the plating we start with a ring-molded portion of mashed potatoes. Surrounding the spuds are a couple of asparagus spears on each side, bent to follow the contour of the potatoes. The palliard is placed on top of the spuds.
Putting any discussion of plating aside, visually it's easy to see how the potatoes and asparagus are support staff for the chicken when done that way. Yet, the total plate is the same protein and two sides.
And, of course, after saucing the chicken, don't forget the hunk of curly parsley.