Information Systems & Technology. They're mostly contractors that are hired to build and work on Apple's internal tools and infrastructure.

Buzzfeed News has an excerpt [0] from Alex Kantrowitz's book Always Day One [1], which contains interviews with former employees that describes the dynamic.

[0] https://www.buzzfeednews.com/article/alexkantrowitz/always-d...

[1] https://www.goodreads.com/book/show/52027218-always-day-one