Mac OS 10.15 or later, defaults command does not create a plist?

Perhaps if you explain what it is that you are trying to achieve with this one can better assist you.

The defaults command is not typically used with a file path, but with a reverse url, for example

defaults write com.apple.safari something -bool true

would affect the plist for Safari (stored in ~/Library/Preferences).

I have never seen the command used in the form you stated above, and I don't see what it is the purpose, so please explain.

I don't know if it can write to a file, though it says it could.

This warning in the man page might be pertinent (though I'm not sure if it means what I'm inferring).

WARNING: The defaults command will be changed in an upcoming major release to only

               operate on preferences domains. General plist manipulation utilities will be folded

               into a different command-line program.

Why not just write the plist by hand or use Xcode to create one?

You can generate a skeletal .plist with the following AppleScript:

use framework "Foundation"
use scripting additions

property NSString : a reference to current application's NSString
property NSMutableDictionary : a reference to current application's NSMutableDictionary
property plistname : "~/Desktop/sample.plist"

set out_plist to (NSString's stringWithString:plistname)'s stringByExpandingTildeInPath()

set dict to NSMutableDictionary's dictionaryWithCapacity:1
dict's setObject:"text" forKey:"akey"
dict's writeToFile:out_plist atomically:0
return

If you replaced the "text" entry with a URL link, the "akey" with the new key "URL", and the plistname with "~/Desktop/sample.webloc" filename, it would write out a .webloc example.