In addition to Android 11 and One UI 3.1 goodies, the update brings the latest security patch update released in March 2021. This is a major update having a download size of about 1GB. So, It is recommended to download the package over a Wi-Fi connection.
The update should bring features like chat bubbles, private share, new UI design, AOD customizations, one-time permission, dynamic lock screen, more lock screen widgets, dedicated conversation section in the notification area, and auto permission reset.
The Android 11 update should reach your Galaxy M21 with a pop-up notification. If you have not received it yet, then check the status manually by navigating to Settings >> Software update.