As for Google's OEM partners that participated in the Android P beta, they will have to wait a bit longer to deliver the final release of the operating system to supported devices. Google writes:
And devices that participated in the Beta program from Sony Mobile, Xiaomi, HMD Global, Oppo, Vivo, OnePlus, and Essential, as well as all qualifying Android One devices, will receive this update by the end of this fall! We are also working with a number of other partners to launch or upgrade devices to Android 9 this year.
Although we aren't holding out breaths, we can only hope that hardware OEMs are quicker about upgrading current devices to Android 9 Pie this time around. As of late July, just over 10 percent of all Android devices were running Android Oreo.
While the Android 9 Oreo operating system is available right now for Pixel owners, some specific features won't actually "ship" until the fall. These features include Digital Wellbeing (which gives you a centralized location to monitor your smartphone usage to help combat smartphone addiction) and Slices (which allows you to access portions of an app outside the "full screen" experience).
These features, however, are available in beta form for Pixel owners and will be available in production form in time for the wide release of Android 9 Pie.
Other features that are already baked into Android 9 Oreo include Adaptive Battery, App Actions, Text Classifier and Smart Linkify, which all relies on AI machine learning. However, there are too many new features to list here, so be sure to check out our initial article on the Android P beta along with Google's current blog post.
For Pixel owners that want to go right ahead and dive in, you can download Android 9 Oreo system images right here.