Scroll halfway down, see "Tip" box here:
Also, further down the page, you'll see this:
Show the completely customized product on your landing page
Link to a landing page that features a specific product, instead of a generic gateway page where users can begin the customization process. Learn more requirements for Google Shopping landing pages
If you put a variant in your feed, have an image for it, and a landing page.
There are a variety of interpretations on this topic, my opinion is to show people the most popular variants and land them on configured products on a dedicated page, so if that is exactly what they were looking for, bam, you give it to them. If you try to split a zillion variants into a zillion products, each won't have enough data, and you'll have too many landing pages to have a navigable website.
One more opinion, if the variant doesn't change the image at all, it probably shouldn't be a dedicated item, but rather is just an option on the base model. On a bike, for example, three bearing choices probs not, three styles of handlebars probs split.
Avoid ?color=black, query parameters are rarely optimum.
Good luck, you've got a lot of hairs to split. And sorry, I know I wasn't definitive, that is the nature of this question. :-)