Trust me: you really don’t want to do this in Big Sur. As explained above, in order to do this you have to break the seal on the System volume. You can’t then reseal it. Ever. That's not entirely true. I for example needed to modify the /usr directory, so I followed the tutorial and, yes, had to leave SIP disabled afterwards. However, you can reenable SIP after you install an official Apple update which tinkers with the root volume. At least that worked for me. I suppose that makes the checksum kosher again, or something. That said, this locking down of the OS is becoming obnoxious. If I work on a desktop, it's also because I need to access the system freely, something that, say, iOS would be woefully inadequate for. By all means, throw up all kinds of dire warnings if we try and do stuff that might be dangerous but please, Apple, don't castrate macOS any further.