How are modern fighters cleaned?
Yes, they use a chemical to clean aircraft. Back when I was in the Air Force, we used PD-680 type II.

I'm pretty sure they don't use that any more though. PD-680 was discontinued (at least for most applications) around the turn of the century. Most applications that used to use PD-680 switched to PRF-680. I can't guarantee it, but I'd guess that's what they use to clean aircraft.