That's the thing - there is no marshaling. The objects are passed between the languages as pointers under-the-hood. There is a standard way of invoking methods/functions on those pointers, that works across all languages.

To give you an idea of how deep this goes - you can pass a Ruby object to a C function and the C function can read and write it as if it was a pointer to a structure.

http://chrisseaton.com/truffleruby/structs/

It has a form of universal object used for scripting languages (but not more static languages like Java or C++). But using it is not required, it's just a convenience to make implementing languages easier.

What it actually provides is a form of "universal abstract syntax tree". So if a C program wants to access a JavaScript object by writing "a->b = c" then the dereference is turned into a snippet of JavaScript code (in effect) and inlined right into the C function.