This example is discussed extensively in the Introduction to pomp and Advanced topics in pomp vignettes. The codes that construct these pomp objects can be found in the data-R directory in the installed package.
Do file.show(system.file("data-R/sir.R",package="pomp")) to view these codes.
For the basic rprocess, dmeasure, rmeasure, and skeleton functions, these codes use compiled native routines built into the package's library.
View src/sir.c in the package source or file.show("examples/sir.c") from an Rsession to view these codes.
The boarding school influenza outbreak is described in Anonymous (1978).