T-Mobile's dedicated page confirms that LG Google Nexus 4, LG Google Nexus 5, Motorola Google Nexus 6, Asus Google Nexus 7 (2013), and HTC Google Nexus 9 devices started receiving the update on Tuesday via OTA (over-the-air) over Wi-Fi only. The cellular operator notes that the new security update for Nexus devices will be available over cellular network at a later date. The pages also note that these are mandatory updates for the Nexus range. Apart from Android security enhancements, all the Nexus devices will also receive minor bug fixes.
For Google Nexus 4, Nexus 5 and Nexus 9, the new Android security update comes with build number LMY48M with Android 5.1.1 Lollipop. For Nexus 6, the update comes with build number LYZ28K based on Android 5.1.1 while Nexus 7 will receive the update with build number LMY48P. Other carriers may have slightly different build numbers. Google has also uploaded the updated factory images for the Nexus 4, Nexus 5, Nexus 6, Nexus 9, and Nexus 10.
Google had announced that it would notify all Android manufacturers about the security issues every month in a security bulletin. It also announced that it would start rolling out OTAs every month focused on fixing security issues for Nexus devices.
The Android updates usually follow a long process before hitting every handset which starts with Google releasing the update to its partner OEMs. The update is then modified accordingly by the company and is seeded to different mobile carriers across the world. The mobile carriers again tweak the UI before finally releasing the update to users. The whole process of an Android update sometimes takes almost three-six months to reach out to the users.