While Phil is right that the phrase "final assembly" describes what is done in a maquiladora, that phrase fails to capture the most important feature of a maquiladora which is the "free trade zone" in which the maquiladora is located. (see the Wikipedia article that Philip linked to) I am not well-versed in the details of import/export duties between the U.S. and Mexico but the key idea of a maquiladora was that raw materials could be imported to Mexico duty-free if they were intended for products that would be re-exported (to the U.S. for example). I think there is also favorable treatment when the finished product is imported into the U.S. (I would imagine that the value of the raw inputs would be subtracted from the value of the finished product so as not to create a penalty for having shipped the goods to Mexico in the first place). Thus, the phrase "final assembly" doesn't convey the full meaning of the word "maquiladora" and so the word most often used in American business is the word "maquiladora". If an English speaker doesn't know what a maquiladora is, then the concept is explained to them.

I've never heard of this, but Wikipedia has :-) https://en.wikipedia.org/wiki/Maquiladora

I would say "(final) assembly" depending on the context, of course.