Big Sur

Sorry to hear that you still have this problem. This seems to indicate to me that the problem is that your internet is not functioning up to par for some reason. Here is what I suggest trying. First turn off your router for at least a minute to reset it. Then see what happens. Next you can go to your internet provider's web site where they often have links there which allow you to test your connection for reliability and speed. Then they also have links where you can connect with their help personnel to trouble shoot problems. After all, you are one of their paying customers. All internet providers are not the same interns of reliability.

Are you using Wi-Fi to attempt the download? If so, don't. Use a wired Ethernet connection to your modem/router. If you are using a download link, only use the Safari browser, not Firefox. And stay off the internet during the download process, which should take an hour, more or less.

You ask very good question. Here is what I know. For most smaller downloads, Wi-Fi seems to be just fine. However, downloading OS system updates and upgrades is another matter. They are normally very large files and anything that affects the download can cause corruption of the very important OS file. Sometimes doing a Wi-Fi download will go just fine. Sometimes, no. When there is file corruption, it either will not complete the download, will not load, o r if it does, you will experience a corrupted operating system. Not good. A wired connection avoids all this. If one is not possible, then sometimes it will be necessary to do a number of lengthy downloads vis Wi-Fi to get a clean one that works. In summary, sometimes it works, but sometimes not. Also, keep in mind that not all internet providers are equal. I assume that not all modems are always functioning at 100%. Simply put, wired is always faster and more reliable.

Use System Preferences : Software Update panel to initiate the update of Big Sur 11.1. That works from first-hand experience.