I thinking the easiest way to connect the Interpreter selectors to the sources code is the IO primitives. These are the easiest to follow.
Beep goes from Interpreter>>primitiveBeep to /platforms/iOS/vm/Common/Classes/sqSqueakSoundAPI.m/int ioBeep()
The primitives are a point to work out from. At the other end of the spectrum is the JIT.
Between the two positions somewhere are byte code interpretation and memory management.
Another simple observation is the ratio of Interpreter/ObjectMemory selectors to C/ObjC functions. It's hast to be 10:1.
A huge amount of effort goes into managing memory. I just read Glen Krasner's Aug. 1981 Byte article on the vm. It's a misnomer.
The article is solely about object format.