First, download Big Sure from MacApp Store.
After the download is completed. The system will be saved into your Aplications folder with the name 'Install macOS [version name]'. The installer will open after download, just close it.
Open Terminall app.
Next plug in your USB drive into your mac and check the name of the drive that will show up in finder.
Having this you can start the creation of the install drive, use this command in Terminall (replace 'MyVolume' with the name of your USB drive):
sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Wait untile it's done. And there you have an Boot USB Drive with mac OS Big Sur :)