I have followed all the steps until I try to download Big Sur through Utilities, I get the message that ‘ Failed writing body’ and the download stop

This is likely because you disk format has been screwed by the reset procedure. I had exatly the same issue and you probably have a good chance to recover soon. Just go back to Disk Utility and check if your volume is named Untitled. If, as I suspect is not the case, format the sub-disk under the top master one. This should rename your disk to Untitled. (Try anything you can until the disc is renamed, you may see some errors and I do not remember exactly what I did)

Once the disk is renamed, please cut and paste each command, line by line to check that everything goes right.

PS: The list of commands were not functioning correctly because the /Volumes/Untitled wasn't existing, leading to a download in the wrong, small place, with a stop at 21%

Good luck :-)