If the localizer or GS needs adjusting a tech will make the adjustments on the towers inside the ILS shack and is basically altering the frequency eminated from each tower to balance everything and bring it into tolerance. These points are surveying using traditional survey methods, and when the FAA comes they basically place their instrument at these points of where the localizer 'should' be and compare the difference.
The other survey you see is ground based where the FAA will survey points on the airfield that are at 10, 20, and 30 degrees off centerline from the localizer. The aircraft will make several approaches to the runway with different on board computers measuring the signal and recording the results. The one that most people see are the flight-check aircraft (usually registered as N1-N99 and are usually King-Air's or Lears). To see how much a localizer or glideslipe need adjusting a couple surveys are done. This is where the adjustments take place. Inside the shack you'll find a few large units that resemble something like a computer mainframe. The cables are fed undergrond and into a small shack (usually referred to as the ILS Shack). As you said, the localizer array is usually centered on the midpoint of the runway (most of the time, there are exceptions) and is made up of multiple towers.
I can tell you a little bit from my experience though. To be honest, unless you've got a degree in physics and radiowaves you wouldn't understand how it's done (don't worry I don't know either).
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