Question: How are built-in types handled differently by Breathe?

[RobertoRoos]

A question about the workings of Breathe. Apologies if this belongs on StackOverflow instead.

How does Breathe handle references to built-in types, compared to user-defined ones?
I am asking because I am making a Sphinx extension for a new language.

For example:

.. cpp:function:: double my_function(float number, MyClass my_object)

Here Domain.resolve_xref() is triggered for MyClass, but not for double and float. Where is the identification of built-in types done?

I noticed that for the PythonDomain, the built-in types are not caught directly, they pass as unresolved references. But only in the missing-reference event they exempted from a warning message.