Well, it just proves my point: Quicktime CANNOT MAKE a multitrack file itself.
When you have to use another software to MAKE a mutitrack file (as shown on that website), in that case Final Cut Pro you cannot assert Quicktime can do it.
Quicktime can only route these tracks to the corresponding channels of a given mutichannel audio card.
So at least my confusion has been eliminated.
I have managed to test your Multitrack Maker on another computer with macOS Ventura. That's what I was talking about: your software CAN combine multiple tracks and save it as a movie file. This is NOT what Quicktime can do.
Thanks for your quick responses. It was a little complicated to get to the meat of it but we got there eventually!