Near as I can tell, it actually depends on what you're buying. If it's for the app itself, like an upgrade to "premium" or stuff for the game you're playing, it has to go through Google Pay and they take their cut. If it's something unrelated to the app, like a physical product or digital content for use outside the app, you're not beholden to Google Pay. But it seems like there are some grey areas.

I didn't get this from any single source, just bits and pieces put together from random forum posts and a few quora threads. And of course policies can change, so anything I found might be different right now. But I know you can buy digital books through Amazon's Kindle app on Android and it doesn't look like Google gets any of that cut: it's the normal "one click" purchase option you see in their web site, no prompt for Google Pay, just click and it's yours, downloading right away.