According to what I have found online the full installer application should be named "Install macOS Big Sur" and appear in my Applications folder even before the full download is complete.
After carefully searching and monitoring, no new files show up in my Downloads or Application folders during or after the installer is downloaded from Apple. No recent files can be found containing the words Install or "Big Sur" anywhere on the Mac. The few files I can find that include the word install in the title are fairly old and under 5 MB (far too small to be considered an OS installer). So, I suspect whatever is downloaded is hidden or stored in an obscure system directory. What I am asking for is the location of these files and what they should look like, so I can try removing them before restarting the Software Update.
When I use the Software Update feature in System Preferences, 12.43 GB in files download within a few minutes, (the progress bar moves in spurts, but rather fast). The first time I tried to update to Big Sur was yesterday. It took at least 30min to have presumably downloaded the whole update with the progress bar inching along the whole time. So, I suspect at least some of the files are cached, somewhere, from the original installation attempt and not being overwritten on subsequent update attempts. It seems the downloaded files are never successfully joined into a single installer package. Otherwise I would expect to see the full installer appear in my Applications folder or be able to locate the file(s) using the Finder's Find function.
I get the same result every time I try to update/install Big Sur: The download happens without any apparent errors, Installation starts after download is complete, but aborts after 5 minutes of attempted installation. The error message I see is always the same: "Installation Failed - An error occurred while installing the selected updates."
I monitored my Downloads and Applications folders during the installer download process and after the download completes. I never see a single new file show up in my Applications folder.
I tried all of the advice suggested above without any effective results. The system indicates the update is downloading, but the update always fails during installation.
I am performing the update on a fairly well equipped 2018 MacMini. It has over 1TB of free disk space and no other software running besides MacOS 10.15.7 Catalina while the system upgrade is being performed. I have run various diagnostics on my system. No CPU overload, no disk errors, plenty of free RAM, no memory pressure or sensor flags. All diagnostics pass without errors.
