No, then you won’t even be able to use in-app purchases.

I didn’t realize that. Never actually tried to buy anything. You can’t even make purchases in the Samsung store? Or Huawei?

OP is mistaken - you can make purchases in side-loaded apps, only thing is that app can't use the Google Play APIs for that (obviously) - but they're free to use PayPal or stripe or w/e payment method. Google has no way of preventing sideloaded apps from doing that, and it's not like they can ban them either.

You are put through a series of warnings just like you are when downloading an executable in the browser, or installing it on Windows.

Actually, there isn't even any actual "warning" - at least not on my Fold 4 - there was just one dialog to enable installation from unknown sources, with a "Settings" button that takes you directly to the page where you need to tick the box next to your browser, and as soon as you tick the box, you can click on the "Install" button to install it. That's it. None of the dialogs you interact with has any actual warnings.