The new build carrying the August 2021 security patch comes with firmware version G99x0ZCU2AUGE, and the update for the S21 Ultra requires a download of about 300MB.
The update is currently seeding in China and should begin rolling in Hong Kong and Taiwan in a few days, with the rollout expected to expand to more countries in a couple of weeks.
If you live in China and haven't received the new firmware on your S21/S21+/S21 Ultra yet, you can try checking it manually by going to your device's Settings > Software update menu.
