Big Sur was riddled with problems from getgo. It bricked some models, started installing when there was not enough disc space available and all kinds of weirness. Quite few of the minor versions had issues.
I was stuck with version 11.0.1 for over half a year. Of course my mac was hit by one of the problems, I tried navigating around the problem, contacted Apple Care (yay! finally got something ouf of that extra money) and they said a full install will be required. When doing the full install didn't work I lost motivation and just ignored any updates for months.
Going for the typical path, macOS of receving a notification about update or via App Store, they are pretty much the same thing, difference being if I actively seeked for updates or was reminded about it. Result:
Ok. Doesn't seem to work. I triied downloading on Wi-Fi, on Ethernet with couple of adapters, then I gave up and went for my favorite way of doing updates, USB-stick. Preparing the stick, booting from it, quite soon I was informed about "A software update is required to use this startup disk".
Clicking Update resulted an instant error:
Preparing something to do something didn't go as planned. Error message of "An error occurred preparing the software update" was emitted. Trying again wouldn't help. Booting into Internet recovery wasn't helpful either, error -2003F indicating failure to access resources over Internet. All avenues were explored, everything I knew was done. I totally lost motivation to attempt anything. All that time the Apple Care chat message "just do a clean reinstall" demotivated me more. I WAS TRYING TO DO A CLEAN REINSTALL!! It just didn't work.
I wasn't alone. Many many many users had exactly the same symptoms.
Macs are funny that way. Apple does "support" their old macOSes for security, but old hardware has a cap on what macOS will be the the last one for that particular hardware. At the same time you absolutely positively need to fall forward into latest macOS versions to have support for all nice software. My Mac wasn't capped by obsoleted hardware, it was capped by Apple's own QA failing and allowing a deploy on an OS version that wouldn't work properly. In this case, I needed to update Xcode, the must-have Mac / iOS developer tool. The version I had runing worked, but didn't support the new and shiny things I needed. I wasn't allowed to upgrade as my Big Sur version was too old.
You Maniacs! You blew it up! Ah, damn you! God damn you all to hell!
(For movie-ignorant readers, that quote is from -68 Planet of the Apes)
Need for a recent Xcode sparked my motivation. I went back to the grinding stone and trying to force my thing to update. I re-did everything I had already done before. No avail. Nothing works. Every single thing I attempted resulted in a miserable failure. To expand my understanding about the problem I read everything I could find about the subject and got hints what to try next. Apparently the problem was with the T2-chip.
For the umphteenth time I did a recovery boot and realized something I didn't recall anybody else mentioning:
Among numerous options to install or upgrade macOS I knew about, there is one additional approach in the recovery menu. And IT WORKED!