The 11.6 update was a maintenance release focused on patching a major zero-click exploit (CVE-2021-30860).
macOS Big Sur 11.6 is a free software update released by Apple on September 13, 2021, primarily to provide critical security fixes. While it is a legitimate and free download for compatible Mac users, you should only obtain it through official Apple channels to avoid malicious advertisements or bundled adware that often use "Free Download" hooks to target users.
To download and install Big Sur 11.6 for free, your device must be on the official compatibility list: macOS Big Sur 11.6 is Out! - What's New?
It addressed a vulnerability where processing a maliciously crafted PDF could lead to arbitrary code execution, an issue known to have been actively exploited. Build Number: 20G165.
Approximately 2.6 GB to 3 GB depending on the device. System Requirements