How really can someone make a private server of a flash game?

I understand that its complicated but I would like to learn the way. So you download the swf file with inspect and then what?

Thank you :)

You'll need at least an emulator to run the Flashgame, or the source code. You cannot just "download the swf" and upload it at 127.0.0.1, I mean you can but it's not gonna work.

SWF-Editing needs mostly decompiling the swf-file itself and edit it with adobe e.g.

If it's multiplayer, you'll need database/server/configs etc.