Which word would you use for something 'internal' or 'in-house'?

Question

There is a word 'kitchen' in my language, like:

• It is their internal kitchen - in this case, 'kitchen' means something we don't care or we should not be aware of, since it is their business and we are good with it

Does it have the same meaning or usage in English?

For example:

• politics kitchen

• management kitchen

• What about them? No worries, they know their business, it's their kitchen

Answer 1

"wheelhouse" is a similar word.

wiktionary
(Canada, US, figurative) A person's area of authority or expertise.
Synonym: domain
Horse viruses are in Pat’s wheelhouse.

dictionary.com
in one’s wheelhouse,
within one’s area of expertise or interest:
There are some subjects that are in your wheelhouse and some that are not.

However, it's usually limited to the preposition "in." You would not say:
politics wheelhouse *
management wheelhouse *

And it's also a somewhat quirky expression that some listeners might not be familiar with.

Another saying:
"We should stay out of it. Too many cooks spoil the broth."

Putting aside figurative words, there are more standard choices:
"That's their specialization / specialty / area of expertise / domain of expertise / job."
"They are experts / gurus / professionals / very knowledgeable."

"So, let them worry about it. It's not our business."

However, none of this matches "kitchen" in an exact one-to-one translation.