Confused. Just run software update, and when the install starts, exit it. Look in the /Applications folder and the installer is there.

Run:

sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

Here is the support link:

https://support.apple.com/en-us/HT201372

I just did it and it works just fine.

UPDATE - I missed the fact it was the point release updates for some reason. Sorry. Leaving post as it is helpful for those that want to make a boot USB.