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.

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!

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.