how do i get macOS version 10.15.0 or later on a 2021 mac computer?

You can't. Catalina doesn't contain the drivers needed to boot Mac models that originally shipped with Big Sur and newer.

(208860)

No Mac will boot from a macOS version from before that Mac was first shipped, Catalina or otherwise.

I haven't seen any vendors offering full operating system emulation support for Mac, either.