URGENT! Unable to restart MacBookPro after a failed Big Sur upgrade. Tech guy suggested erasing the HD

Hi,

Two days ago I tried to upgrade the OS to Big Sur. The system froze during the upgrade (black screen, apple logo, "less than a minute remaining..." for hours and hours).

I tried to shut down the macbook and restart via:

1) command + R

2) command + option + R

3) command + option + shift + R

1) and 2) took me to the screen asking for the computer password (the one i use ten/twenty times a day to unlock my macbook - i.e. the password I surely remember -- but could not log in as if the credentials were wrong.

Option 3) took me to Disk Utility and I tried "First Aid" and Erase. All came back with "failed" error messages. I tried to reinstall the original OS, Sierra, and got an error message as well.

When I simply shut down and restart the macbook it brings me back to the Big Sur installation screen (black screen, apple logo, "less than a minute remaining...").

I took the computer to a tech support place in my town and was told the only option i had was to reformat the hard drive and to lose all the data that is not stored on Cloud. I was even willing to do that but turns out even this option has failed. The tech guy called a few hours later saying there is nothing he can do. His explanation was that there was not enough memory on the hard drive to install Big Sur and, currently, the lack of memory prevents the computer from starting at all.

My macbook is only three years old.

Please help. Right now i am unable to use the laptop and struggle to believe that an attempt to upgrade the system could result in such a complete failure. It is unacceptable that Apple would allow an installation of a new operating system without checking for sufficient memory resulting in a fatal outcome.

many thanks,

Oksana