> How does Pd speaks to the GUI/Javascript?

Over a TCP socket connection.

In C there's a generic interface for sending ints, floats, strings,

and arrays containing them.

In s_inter.c I scoop all that up into something resembling a list

of comma-separated arguments to a function.

In pdgui.js there's about 20 lines for a parser that formats all

that into function calls and evaluates them.