There are no ads in the AppleTV operating system itself.
The only Apple “ads” I ever see are inside the Apple TV+ app (yeah, their naming is confusing…) and it’s only for TV shows they’re promoting in their streaming service.
Apple TV is a huge Apple TV+ ad in itself. I shelved my device when my 2yo had "subscribed" to Apple TV+ by just randomly clicking around
I installed an AppleTV recently, so I don't have much experience. But the first thing I saw after the initial setup was one/third of the display advertising a TV-show on a subscription service I had to purchase. Would that count as an ad?