TLDR: Open Terminal and run this command

curl http://17.253.55.201/content/downloads/50/49/001-79699-A_93OMDU5KFG/dkjnjkq9eax1n2wpf8rik5agns2z43ikqu/InstallAssistant.pkg -H "Host: swcdn.apple.com" -o InstallAssistant.pkg

After downloading, you can locate the downloaded file by this command

open .

You will see InstallAssistant.pkg file, just run it to begin the installation.

I’ve downloaded, it’s 12182587199 bytes, MD5 hash 0030d26c6e5d01b7aa10a91b044d6ca7

After installing the downloaded package file, you can open System Preferences > Software Update and click Upgrade Now

To make the USB Installer, you can find the Install macOS Big Sur.app inside /Applications. Or use https://mothersruin.com/software/SuspiciousPackage/get.html to extract the files from InstallAssistant.pkg

Basically the Software Update wants to download InstallAssistant.pkg from Apple CDN, but probably their CDN network has problems. So I’ve tried all the Apple CDN IPs and get one server working – 17.253.55.201

All CDN IPs are:

– 17.253.55.209
– 17.253.57.202
– 17.253.57.205
– 17.253.55.201
– 17.253.57.203
– 17.253.57.207
– 17.253.55.205
– 17.253.57.201
– 17.253.57.209
– 17.253.55.202