AR Stickers aren't a separate camera app on your Pixel. Instead, it lives right inside the Google Camera app. You access it just like you would Portrait Mode, by tapping the Menu button on the side of the camera and selecting AR Stickers as an option. Once you're in this camera mode, you'll see the standard video and photo button options underneath a host of augmented reality characters for you to drop into your shot.
I took the below video with one of the AR stickers. First impressions are it's pretty good!