It has to be from an app. Nothing in Samsung's OS generates ads with few possible exceptions enabled by the user. Samsung wallet, goals and possibly health. Maybe there are others but they are usually promotions that you opt to receive. As I suggested before, you need to look at your installed apps because the OS does not generate ads on its own.
It could also possibly you unknowingly subscribing to a website sending you notifications. I've seen it happen to many people including my mom.
