Technically that Home Screen can display an app’s featured content. And Apple and other apps often use that space to promote new shows. But you can turn off TV+ promos and or move apps out of that region.
Roku actually installs physical ads onto its damn remotes. I have remotes that promote streaming services that don’t even exist anymore.
You can turn that off pretty easily. The AppleTV+ app has the ability to configure how it uses that little quick look space. You can set it to only display your “up next” content.
Any apps in that dock region can show featured content when they are highlighted by the user. Any app can do that, not just Apple’s. Quite a few third party apps use that space to show featured or watchlist content.