Create a MacOS USB installer without already having access to a macintosh machine?

I'm trying to install Big Sur on an HP Elitedesk 800 G4 by following this guide: https://www.tonymacx86.com/threads/big-sur-on-hp-elitedesk-800-g4-g5-mini-the-perfect-macmini8-1-hackintosh-opencore.306132/

It says to create a MacOS USB installer by following instructions here: https://support.apple.com/en-us/101578

It appears that I have to first have access to "a full macOS installer". I don't have one. Are people able to make MacOS USB installers without already having access to a Mac?