that's... I had no idea. Thank you for this insight!
yeah, a shitload of the actual complexity comes in actually getting all the numbers down properly and making a setup that can be manufactured reliably, but guess what, as a writer that's the part you can and
should gloss over, the mangaka may need to consult some diagrams for reference but if it's just text you can just say "they burned some number of months poring over these details, and the yield of jaw chucks suffers but the other components are mostly up to par, and they're now figuring out exactly what the fuck they can do with their new albeit kinda shitty lathes... like making jaw chucks more reliably."
like, one increasingly irritating thing for me (this old man's outgrowing the fucking manga genre at the age of 20-something LMFAO) is that a shitload of manga
do not do research. Black lagoon is wild with its action scenes but shockingly enough it has more realism in it and its politics than some fucking "realist isekai hero" bullcrap.
the simplest electrical motor? magnetize a ring of iron via taking a big coil, put the iron ring/core in it, run current through coil, let it sit for a while (ensure enough resistance in coil so that the thing doesn't burn out from too much current). put magnetized core on a shaft, wrap a number of smaller coils and arrange them around the shaft, and you can use capacitors or more commonly some inductors to split a DC current (like from a shitty voltaic cell) into two alternating flows and from there get a shitty electric motor running by linking these flows to different sets of the small coils. I can't remember the specifics, so a lot of trial and error may be involved, or to begin with some number crunching, but that's the gist of it.
basic computers? even fucking easier, assuming (big if, to be fair) that you have an electrical kit on hand. Mechanical relays, aka an electromagnet influenced gate (magnet on = bar of metal pulled up, disconnecting one side from other, magnet off = bar of metal falls down, connecting both sides, for example), combine these relays into various logic gates, combine logic gates for temporary storage, make capacitor arrays for more permanent storage or use physical medium (see-saw connector/relay, mayhaps?), and you have everything you need to make a calculator and then some. No fancy gear-clutch mechanical computation bullshit.
not to mention factories, and factory scale. Iron plates? if i have a forge setup that gives me suitable ingots, i can do some successive hot rolling to get it down to the thickness I need. Wires? Successive coiling from a thin ingot. Specific cutouts or masks? Take the hot-rolled plates, put them through a punch-die system. Some weird blocky object? Take the ingot, have a worker affix it to some marked position, and then get a bunch of drills to go to work on it. Some really weird object, where cutting ironically ain't gonna cut it? divert some of the molten steel to a special cast molding setup, use a template mold and go ham.
From there, you can start working on PID controllers, larger and more reliable motors, larger accumulators/supercapacitors and power distribution systems (and three phase power), conveyor belts, etc. And there's a shitload of stuff you can write about, here, but all the fucking nerds who could possibly write about it have gone somewhere else, and i'm left behind. Gaahhhhh.
edit 2:
hell, bullets (well, the gun nerds insist that they be called "cartridges"), the casing is a 10 step punch die process, give or take. If you don't care about optimizing it for mass production, and you have the metal and casting/cutting tools on hand, that's maybe a few weeks of work to get a shoddy setup putting out OK casings. Definitely so if you have someone with good hands helping out. the MC here could get to this point in a fucking hour easy (if he knew the compositions for a good primer and for gunpowder, d'oh), and terrify the entire world with mass produced pistols.
When a writer glosses over chemistry, i get it, cuz efficient chemical processes are a pile of all sorts of tools and vat-shaped tools and instrment-shaped vats and even the diagrams make me go "ayo wtf." Electronics, well that's a part of my field so i understand it's easier for me than for your typical writer (still gets absurd sometimes, but generally not
that absurd before we resort to having a computer pack it for us and save us brainpower). But the basic mechanical stuff, there's a lot of engineering and reliability that makes it difficult in practice, but
conceptually a ton of the stuff is not actually that complex.