> Then what is the lowest macOS version the binary can support??? It's very tricky [...]
No it's not. The oldest supported version of macOS your binary will run on will be the one you set in your CFLAGS with the -mmacosx-version-min=10.x flag when you build.
The OS you are running the build on and the Xcode version don't influence that, you should just run the most recent Xcode you can, build against the most recent macOS SDK available and set that flag. You can target 10.8 from Big Sur and Xcode 12 with afaik no issues.