i am not sure if he was the originator, but for years the industry standard of Nut/ Bolt & Nut, Bolt, Washer styrene castings was Cliff Grandt, d.b.a Grandt Line Products. finest HO scale window castings & architectural details, too.
he passed away and subsequently the line was purchased by Tichy, now Tichy Train Group. they may have scooped up a couple of other manufacturers on the wane.
the castings, available in a plethora of sizes & styles, just need a steady hand & hole to plug in to. a dab of adhesive (depending on what's being detailed but always styrene compatible) and on to the next one. expect some loss to the dreaded carpet monster, but that's half the fun.
other companies make them. i believe RMCM has some, and oodles of military model manufactures. some are just the heads on a sheet of styrene and you slice them off with no hole needed for application. you can also make your own nuts & washers separately using Plastruct rod (hex, round, rod in graduated size) by bologna slicing and stacking, but i'm too old for such shenanigans.
if you've mucho dinero, go for tiny brass.
check out some videos: