I occasionally see a small ad near the top of "Settings", usually for an Apple service. I don't recall ads on the home page or lock screen, or in the main Apple apps I use (Camera, Photos, Files, Health, Fitness, Watch).
Well on my work Mac I'm not signed into iCloud, and there's a permanent red notification dot, just as prominent as if I had an important system update waiting, begging me to create an account.
The only time you get those notifications is generally when you install a new os version and they want to tell you about a new feature. That feature might be 'hey we launched apple news that you can pay for'. The only other time is when you are close to your apple cloud limit it will tell you that you can subscribe to more space.
Where are you seeing ads in the main UI?
I occasionally see a small ad near the top of "Settings", usually for an Apple service. I don't recall ads on the home page or lock screen, or in the main Apple apps I use (Camera, Photos, Files, Health, Fitness, Watch).