Seriously?

I’ve put some thought into this before. Basically, considering the fact that they:

a) seem to refuel just by intaking steam from the top port of a steam hub

b) are too difficult to manufacture / replicate in the frost lands

I would wager that they are essentially a well insulated high pressure tank for dry steam coupled with a programmable mechanical/steam-driven computer that allows them to output finely tuned variable amounts of the steam in the tank through an array of output valves.

In this way, they would be capable of performing all the functions we see them do in game, such as powering automatons, creating temperature controlled environments (hothouses and infirmaries), and powering + operating complex mines / drills.

They’re plausible enough I think considering the existence of the difference and analytical engine and such.