It's valid question for people unfamiliar with the project, but it is the AOSP in terms of looks, GrapheneOS does not customize the UI in any way beyond what their own features require as additions. Note that Pixel OS is not AOSP. The default home app of course also influences the experience quite the bit unless you replace it, which is what I'd personally recommend everyone to do as it's so incredibly barebones. Lawnchair is already a big step up as an open source alternative.