How to save an Arcade extension to hard drive?

I know that if I save an Arcade game that I will be saving it with installed extensions. I know that if I need an extension I can go to Github online where it is stored and download it into Arcade. What I want to do is save every extension that I have used as a file on my hard drive IF I then can install it in a game “from file” in the extensions page in Arcade. When I download an extension from Github as a file it is compressed and saved as a zip file: and the Arcade environment can not open this. I have tried un-zipping it but still no luck opening or installing in Arcade. So, what’s the trick ? How can I save an extension to the hard drive which will install when off-line ?