In the Traveller universe there has to be folks both working downside
and upside that do maintainence on very comlex systems on starships to
keep them running.

How are they referred to?

On board a US Navy ship you had technicians that maintained the
computers and you had enginemen that maintained (you guessed it) engines
except there were folks working on the main power plants (we had 4) and
ship's propulsion systems (gas turbines) collectively referred to as
engineering.  My father in law was referred to as an engineer not a
mechanic because he kept the high pressure steam generators working in
Boston. FIL did not have an engineering degree but nonetheless was
referred to as an engineer (officially Stationary Engineer)

How do such titles map in Traveller?   Some of the systems a starship
use would be fairly complex requiring a high degee of education but
still you'd need "grunts" to turn screws and tighten large nuts and in
some cases weld up hull plates.