How to Install Android 12 on Oneplus 7 & 7T Series.
The fourth beta release of Android 12 goes live today, and it marks the Platform Stability milestone. This means that Android 12 Beta 4 has finalized APIs and app-facing behaviors, so developers can test their apps on the latest release without having to worry about changes in the next beta impacting compatibility testing.

Three methods to install the update are:
Method 1: Sideload Android 12 Beta via Recovery and ADB
Method 2: Flashing full Factory Image via Fastboot
Method 3: Install Android 12 Beta GSI
Prerequisite :
Commands :
fastboot flash boot_a boot.img
fastboot flash boot_b boot.img
fastboot -w flash system <your Treble ROMs name>.img
fastboot –disable-verity –disable-verification flash vbmeta vbmeta.img
fastboot reboot