built-in and inbuilt

The laptop has got a built-in (or inbuilt) modem.
Does both go well in the above sentence?
Can they be interchangeably used in all contexts?

Is this sentence correct?

“Since no Headcounts were approved for such initiatives, HR officers are positioned against Zero Head Count.”

I don’t think so (in all contexts).

Can you explain, Haihao, maybe, by giving a sentence where one fits in and the other does not?

Built-in: Constructed as part of a larger unit; not detachable, etc.
Inbuilt: Built-in, and inherent.

For example, Polarity is inbuilt (= inherent) in a magnet. But you cannot say: #Polarity is built-in in a magnet.

Yes. Polarity is built-in in a magnet doesn’t make sense.
Thank you.