Why use the prep “to”, why not others like “of” in the following sentence?

Question

There are three fundamental parts to the <locale> header.

Answer 1

Idiom? The following two variants are pretty much synonymous:

There are three parts to the header.

There are three parts in the header.

The reason for this is, pretty much, just because those are the constructions allowed by English. The use of in follows pretty closely the locative meaning of in. The use of to is licensed by a general English idiom "There is X to Y", meaning "Y consists of X."

However, I find the following to be slightly awkward:

There are three parts of the header.

I suspect that the reason is because we expect the trailing prepositional phrase to be a sentential prepositional phrase and not part of a noun phrase, and of is never used to head sentential prepositional phrases.