To be honest there is not much to it, you buy the movement, put it in a case, and put the hands on it. you can get everything from aliexpress. it's easier and often cheaper to just buy a normal watch if you need one.
buying movement is like buying whole PCB
DIY analogy would probably be about acquiring individual gears
Is it different with a smartwatch? You buy the kit, it's not like you solder much as far as I understand.
It's impressive you start with a lathe and make the movement yourself!
Not nearly as impressive as designing and fabricating your own integrated circuits and display!
I thought so too, but after quick research apparently there are kits. For various values of "DIY", I guess...
Sure but buying a movement kit is no different than buying a pcb. Writing code is not impressive any more.