How do I view the built-in definition of TemplateBox styles in terms of low level boxes?

The following:

TemplateBox[{"a", "b"}, "Binomial"] // RawBoxes

yields the correctly formatted (2d box structure) of Binomial[a,b]:

$$\begin{pmatrix}a\\b\end{pmatrix}$$

But I would like to see its (and other built-in functions') explicit definition in terms of low level boxes. Presumably, the definition of the style "Binomial" lives somewhere. Where is it?