"automated" is the wrong term to describe the Drummer. You have think of it as two components.
Drummer - the drum pattern generator
When you load a Drummer Track, GarageBand creates that special Track that includes a Drum Pattern Generator, which "automatically" creates drum grooves based on the parameters set in the Drummer Editor. When you load different "Drum Kit" Patches from the Library, you just swap different Drum Samples.
Drummer - the sounds
This is the actual Software Instrument (it is actually an EXS24 Instrument), the sampler that has all the drum samples loaded. When you load those those Patches to a Dummer Track, then the Drum Pattern Generator on that Track uses that instrument. You can load those Patches to a Software Instrument Track and record your standard MIDI Region to create your own grooves using those Sample Instruments.
I explain all the details about the Drummer in my graphically enhanced manual "GarageBand X - How it Works"
Hope that helps
Edgar Rothermich
http://DingDingMusic.com/Manuals/
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