Release notes for Android15 Beta 4.2 (Build AP31.240617.015) adds the new devices on August 22
Pixels 9 listed on the Android Beta Program website as eligible to receive the beta version + OTA installation | Image credit — PhoneArena
Once you've enrolled in the beta program, you should receive an over-the-air (OTA) update for the beta. However, keep in mind that beta software can be unstable and contain bugs, so it's recommended that you back up your data and be prepared for potential issues before installing the beta.
As Android15 As the official release draws ever closer, we can expect Google to continue refining the software and fixing any remaining bugs. Future beta releases will likely bring further improvements and optimizations, and may also introduce new features that weren't included in the first beta.