This is a very tiny program (8 KB). Here is an excerpt of the original ReadMe file (also included): Welcome is a simple application which allows you to edit the “Welcome to Macintosh.” message that is displayed when the Mac boots. Welcome comes with no warranty, use it at your own risk.
system FILE, not system FOLDER, there is a file in the system folder called System, it has a suitcase icon
edit: I didn't read all the way, I think you make it yourself
It's a file you can create yourself, using apps like ResEdit or MacPaint. It basically is a file with a 'PICT' resource with ID 0. I attached an example for you. Just extract it and put it in your System Folder.
I found that the starting screen of the production can only be full screen, and the 7 system can not be a full screen. The system below 7 must be full screen