Before people get too up-in-arms about that being XML, that is just the first step in the GObject Introspection compilation.

The second step builds a .typelib file which is a mmap()'able data-structure used at runtime by dynamic languages. It's smaller as it doesn't contain things like function/class documentation.

Additionally, the language bindings tend to use the GIRepository base structure which handles loading those mmap()'able data structures with an C API on-top of it.