Like many frustrated users, I've been trying to install Big Sur for many hours. I'd post a screen-grab of the failure but somehow, I think that's already been done. In my case, the download went just fine, then hung at "12.18 GB of 12.18GB" before failing after 5 min or so. I've solved the issue by uninstalling Avast Security (free version) and rebooting; Big Sur installation proceeded at the first attempt. At no point did Avast notify me of anything and I can't comment regarding other 3rd party security software, but hope it works for you too.