The Substack model is that you provide both content and audience, and they provide hosting and monetization tools. They're a service provider where only the author is their customer, and so they're able to focus exclusively on what the authors need.
This doesn't happen in practice. Like most marketplaces they have an incentive to market things that are popular or appear to be trending. That means in order for Medium to promote your article you need to do the marketing with your own audience or outlets to get eyeballs on the article and after Medium flags your article as popular it might get promoted.
This is a really big concept because it means if you're just starting off with no audience you might as well grow your own audience on your platform because most marketplaces like Medium (and others) aren't heavily promoting folks with no prior audience.
I remember putting some of my blog posts on Medium and setting the canonical URL back to my site. Out of the hundreds of thousands of visits my personal blog got I only had a few hundred total hits on Medium. That's because I didn't promote and drive my own traffic to Medium and in turn they didn't promote me.
This resulted in being able to build an audience that would read my work irrespective of whether I published with a publication.
That's not something I saw when I used Medium in 2018ish for some of my posts.
I know there was a partner program but that wasn't paying to get traffic to your posts. That was a mechanism to get paid based on how much engagement you had on your Medium articles. It's just another incentive to get you to drive your own traffic and audience to Medium to drive up Medium engagement. Basically helping Medium grow their business.
If you wanted to pay money to promote your content you can do that without Medium. You can buy ads on Google and other outlets then run ads to your blog posts. I don't do that personally but it's an option.
That suggests that Medium should be more reader-friendly than Substack, but it seems the opposite is true. I suspect that even when readers are nominally customers on Medium, they are really still just the product (in the sense of “if it’s free, you’re the product” — except it’s not free here).