How do PostScript interpreters know the meanings of PGF-specific operators, such as 'pgfe' and 'pgfstr'?

The PostScript definitions for pgf/TikZ can be found in the files

pgfsys-common-postscript.def and pgfsys-dvips.def in the TeX directory tree under

texmf-dist/generic/pgf/systemlayer

/pgfstr is defined in pgfsys-dvips.def (as well as again for the vtex version)

/pgfe is defined in pgfsys-common-postscript.def

Definitions for the other output 'engines' like .pdf etc. can be found there as well within files of similar names.

Here is a PostScript function which defines all pgf functions:

/pgffunctions {
    /pgfsc{}bind def% stroke color is empty by default
    /pgffc{}bind def% fill color is empty by default
    /pgfstr{stroke}bind def
    /pgffill{fill}bind def
    /pgfeofill{eofill}bind def
    /pgfe{a dup 0 rlineto exch 0 exch rlineto neg 0 rlineto closepath}bind def% rectangle
    /pgfw{setlinewidth}bind def% setlinewidth
    /pgfs{save pgfpd 72 Resolution div 72 VResolution div neg scale 
      magscale{1 DVImag div dup scale}if 
      pgfx neg pgfy neg translate pgffoa .setopacityalpha}bind def% save
    /pgfr{pgfsd restore}bind def %restore
    userdict begin
    /pgfo{pgfsd /pgfx currentpoint /pgfy exch def def @beginspecial}bind def %open
    /pgfc{newpath @endspecial pgfpd}bind def %close
    /pgfsd{globaldict /pgfdelta /delta where {pop delta} {0} ifelse put}bind def% save delta
    /pgfpd{/delta globaldict /pgfdelta get def}bind def % put delta
    /.setopacityalpha where {pop} {/.setopacityalpha{pop}def} ifelse % install .setopacityalpha 
    /.pgfsetfillopacityalpha{/pgffoa exch def
      /pgffill{gsave pgffoa .setopacityalpha fill 1 .setopacityalpha newpath fill grestore newpath}bind def
      /pgfeofill{gsave pgffoa .setopacityalpha eofill 1 .setopacityalpha newpath eofill grestore newpath}bind def}bind def
    /.pgfsetstrokeopacityalpha{/pgfsoa exch def /pgfstr{gsave pgfsoa .setopacityalpha stroke grestore newpath}bind def}bind def
    /pgffoa 1 def
    /pgfsoa 1 def
    end
} def