The way it works is that the CRT is manufactured with a membrane fitted to the front of the glass. This membrane has the touch sensors embedded in it which communicate with the necessary touch screen software. Some touch-screens can simply replicate a mouse meaning that third party software is not required.
Back in the old days there used to also be things called "light pens" which were electronic pens that had a light sensor fitted to the end. These were connected to the computer and with certain software you could use it for pointing, drawing, plotting etc right on-screen. The way they work is by detecting the position of the pen based upon the CRT sweep sequence and the signals sent back by the light sensor. They didn't require any modification to the computer or screen, just the light sensor and software. I don't think anything like this was available for the Mac but maybe there were light pens about for the Apple IIs. I have used light pens on BBC and other Acorn computers and they're good fun!