Today is Android 13 day, and in addition to Pixels picking up their first stable release, Google is also pushing out Android 13 today to AOSP (the Android Open Source Project, or Android's source code). That means all the code that constitutes Android 13 is now available for other people and companies to play with, and we'll likely see ROMs announce their plans to release Android 13-based versions very soon.
This simultaneous release of Android 13 for Pixels and AOSP isn't always guaranteed. Last year, Google dumped Android 12 to AOSP two weeks before Pixels got the update. But the year before, Android 11 hit Pixels and AOSP within a day of one another. Because Google's Android 13 security bulletin indicated a September patch level, most of us were expecting that we might see an AOSP release happen this month, while an actual update for Pixels would probably be delayed until next month. Fortunately for everyone, it looks like those expectations were wrong.
If you aren't familiar with the terminology or the impact of this news, the abridged version is that Android's code is open source, which means that it is publicly available for anyone to play with. Now that public code has been updated to include the latest Android 13 changes. This source code, stored at the Android Open Source Project, doesn't directly mean that your phone will be getting Android 13 now, but it does mean that more companies and projects now have the details to refer to, and this can impact a lot of things, like the development of custom ROMs based on Android 13, improved app development for the platform, and wider scrutiny of the code for bug fixing and security.
This also means that the Android Beta Program for Android 13 is likely over, but we may see the return of the QPR beta tests for more minor platform revisions and quarterly feature drop updates.
There are a lot of big and small feature changes in Android 13, including a new notification permissions setting, a new photo picker, per-app language settings, better multi-user support, Bluetooth LE Audio support, and even some small visual enhancements to Material You, like a new media player (with a delightfully squiggly playback indicator). It's not the most substantial Android update on a pure volume-of-features basis, but much-welcome polish for last year's big Android 12 update.
Android 13 will land on Pixels today in an update that should start rolling out soon, and other companies, including Samsung, HMD, OnePlus, and Motorola, will be rolling out their own updates later this year.