???
It's almost all done by contract, usually by the same companies that built the equipment. Most of the contractors are former military.
From basic firearms, through the most complex weapons systems, nuclear carriers, satellite systems, etc., they were all built by civilian companies. Adding maintenance to the contract is standard operating procedure.
I have a very good friend who was deployed as maintenance on one of these installations in Montana through his whole hitch.
https://time.com/6212698/nuclear-missiles-icbm-triad-upgrade/
In fact, he was personally there and witnessed the Malmstrom AFB UFO incident in 1967.