Big Sur bootable installer cannot be created or corrupted after creation
I ran into a similar issue when trying to create a bootable installer for Big Sur as well. I found this page to be helpful:
https://mrmacintosh.com/big-sur-installation-failed-an-error-occurred-while-installing-the-selected-updates/
Basically you open Safari and access this URL to directly download the installer:
http://swcdn.apple.com/content/downloads/50/49/001-79699-A_93OMDU5KFG/dkjnjkq9eax1n2wpf8rik5agns2z43ikqu/InstallAssistant.pkg
For me, it would pause every minute or so, but I just kept clicking resume and eventually I had downloaded the whole thing, and I was able to generate a working bootable installer that has worked on at least 2 macs for me.