Why does my PAID Apple TV subscription have ads?

What kind of ads?

The Apple TV subscription would give access to Apple Originals, which is just a section of the Apple TV app. Those subscription movies and TV shows will be ad-free. They may show (skippable) promotional clips for other Apple TV subscription shows at the beginning of episodes, though. They may suggest other Apple TV subscription shows at the ending of episodes, to possibly watch next.

Depending on device, the Apple TV app itself may show areas with content from Apple’ media store or select other streaming services, but not forced in right before, during, or after movie or episode watching.

Depending on device, the Apple TV app may bring access to other streaming services that may not be ad-free. Those would not be influenced by having an Apple TV subscription.

apart from ads for their own stuff, my appletv+ have no ads

about ads for their own thing then even the public broadcasting radio and tv in my country have ads for their own programs.

this is how you cancel the subscription If you want to cancel a subscription from Apple - Apple Support

if will still show up under subscriptions with a renewal option until the free period runs out and then it will be gone.