I tried to edit my post above, but was prevented from doing so. Here is the above post, plus what I had added to it after having posted it.
Hi People,
Is everyone replying in here referring to their devices being "bricked?" I am sorry if this has happened for anyone.
After releasing the update, Apple has now offered some Support Articles in regards to updating to Big Sur and what it would entail. Unfortunately, it may be too late for some or all of you for them to help, but read them to see if there is any other recourse besides taking your devices in for repair, or replacing them altogether.
Look here for the Apple Support Articles that concern updating to Big Sur. Please note that some, if not most, are dated after the update was seeded to the public: https://support.apple.com/kb/index?page=search&q=big%20sur&product=&doctype=SITE_HELP,PRODUCT_HELP,LEGACY,HOWTO_ARTICLES,TROUBLESHOOTING_ARTICLES¤tPage=1&includeArchived=false&locale=en_US&src=support_searchbox_main&type=organic
There were a few things to consider prior to making this major update, which had not been mentioned, had been but it had been several months prior to the release, or just after the release was made public. There is a major change to how Time Machine will work, most notably that its filing system is changed to APFS from HFS+. There is a Support article in regards to that change, but again, it is either newly added, or was added months prior to the release of Big Sur. I will tell you that I am erasing/reformatting my Time Machine drive into APFS prior to doing my update: starting fresh backups.
One also had to considered, whether there were any 32-Bit Apps or 32-Bit scraps still hanging on, even after having had Catalina. Those items can either allow the update to carry on, but render 32-Bit permanently unusable once Big Sur is installed. The other problem that can or may occur, is failure of the update to continue.
One other thing to do for yourself, and this is something that I personally choose to do, please do not leave a major update unattended for any length of time in which your device can go into sleep mode. I know it is sometimes necessary to take a washroom break or get some food. The thing that I always do for major updates or when transferring large files from one drive to another is, I go into System Preferences>Energy Saver, and uncheck any boxes that would allow my computer to go into sleep mode during the process. One box in there to uncheck is, "Prevent Computer From Sleeping When Display Is Off." Once you are done doing the update, just go back and put your preference back in place. As I said, these are things I do on a personal level.
I implore you to please check the Apple Support articles in the link provided, and see if anything in there will get you back on track and/or your device "unbricked."
I hope this helps some/most of you and I wish you good luck!