Contractors, contractors, contractors the US uses lots of them for shop maintenance.
Those aren't actually calculated in that number, as the USAF isn't as addicted to the contract support as the Army, USMC or USN.
When you look at the numbers I pulled from CBO, the indirect PY are your support infrastructure for maintenance etc.
The Squadron size is 353 which is the Pilots, direct maintenance, Operations etc.
The 1,287 is Indirect - which will mean 2nd and 3rd line maintenance, shares of fueling, tankers and other USAF assets to assist in the direct mission, USAF Security Forces for the home base etc.
Overhead is where it gets weird, as there will be a portion of USAF Security Forces for deployed operations, C-17 support for movement, the pay, contracting etc folks who keep the DoD moving etc.
Also when you add up Indirect and Overhead from CBO you will see that there is some tomfoolery in a way, as some units get double counted as they have their own direct unit that is accounted for - but that unit is also used for Indirect and Overhead - to wit, when I started adding PY up I got 207% of the USAF authorized PY
(which I would assume is explained somewhere else as to why that is but...)