Search for "macOS Big Sur" directly in the Mac App Store .
This version was the first major point release for Big Sur and introduced several improvements: How to download and install macOS - Apple Support advertisement macOS Big Sur 11.1 (20C69) Free Download
Apple provides all macOS operating system updates at no cost. If you see an advertisement or a third-party site asking for payment for this specific download, it is likely a scam or a "pirated" distribution that could contain malware. Search for "macOS Big Sur" directly in the Mac App Store