They're in a few different places, but it will sometimes show ads in the Start Menu for apps you can buy on the Windows Store. It also nags you quite often to buy a subscription to Office 365 or OneDrive. You can turn most of the ads off:
Microsoft does a lot of cross-promotion: e.g. the weather app that upon first run shows an ad for the news app.
Under the default and recommended settings, it sends telemetry, and occasionally asks you to rate how likely you are recommending win10 to a friend.
When you launch Firefox or Chrome for the first time, a popup appears from the start bar, saying something like "Have you tried Edge? Edge is XX% faster than firefox/Chrome".
Basically you have to be prepared for a new product placement everytime you launch a Micrsoft app for the first time.
I've related on reddit that psychologically, this feels like the difference between being in your own house and being a house guest of an old acquaintance trying to get you into their MLM scheme.
Where are all these ads that people complain about? Is it the half pre-installed apps that come with your computer that you can promptly uninstall?