> Shopify does have what they call "Shop" now, which works like a marketplace, but in my experience that's still a relatively small channel for Shopify sellers.

It's getting pushed harder due to things like Shop Pay being a thing. I've actually been trying to use it more, given I do purchase an increasing number of things from Shopify-powered retailers - but their search and targeting still needs a lot of work for it to behave like a proper marketplace.