The files app has built in ads to push OneDrive to the users. And yes it can be declared as advertising because you can't remove that ad and because MS pays Samsung for it (or gives Samsung benefits for it inside Windows on Samsungs computers).

Besides that there are the same kind of ads in the settings app.

And there's also many components of the OS that push the user into paying money like for fonts or for Always on Displays or lockscreens or whatever else. Those are not direct ads but you could declare them as auch because of the fact that a user gets pushed into micro payments for basic customization of their phone which for me personally is a no go, especially if you pay money for a flagship phone.

I answered on another comment regarding ads in this thread to clarify what I define as an ad with that comment.