99% of rapid fire mods you see will be fairly basic electronics -- the buttons are usually just switches, occasionally two stage switches, and if you apply a rapid pulse (or not so rapid) or a precisely timed sequence you have yourself rapid fire. Occasionally you get lucky and the thing you want to mod has nice debug ports you can instead pulse with a voltage, and some might even add their own thing at the signals end (mainly for older systems), if your chosen system lacks that then you get figure out somewhere else to put your pulse (most likely directly onto a trace or a pin of a chip, neither of which are especially fun but still quite doable). Some things had to go a bit further -- if you have a really regular signal then it might trigger anti cheat, you then need to find a way to make a less regular pulse, and some things like gears of war auto reload will want a timed pulse. This bit further stuff is marginally harder but still about the second hour of you learning electronics.
The other 1% usually being mods to a game instead and usually done on systems we already have fully exploited (mainly for things that were once arcade games in my experience).