Vim highlight groups definition

In brief, where and how are vim highlight groups defined?

In more detail, and what I'm not asking...

I'm not asking how to find out what groups are currently defined.

I'm not asking how to define a group, which it seems to always begins by extending an existing group.

I'm not asking how to map syntax to highlight.

I am asking how the built-in highlight groups are defined. Are they in fact actually built-in, or is

there some configuration file somewhere that begins with a blank slate and defines them from

scratch?

Kind Regards

Rob.

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