What does the term "turnkey" means or refer to in the Engineering/Manufacturing environment?
"Turnkey" means that you only provide documentation to someone else to build something.  They then source all the parts, get other parts modified or made, etc.  Basically you send them specs and money and they send you fully built units.

Note that this doesn't really apply to PCB fabrication.  Or more accurately, PCB fabrication is basically always turnkey anyway, so that term is not used in that context.  You don't provide copper sulfate, laminated FR4, or etching solution to the PCB house.  You provide only Gerber files (the documentation), and they do and buy whatever is necessary to make the complete finished boards.

What you are probably confused by is that some PCB houses also offer assembly services.  Those could be turnkey or not.  But that's really a assembly issue.  Someone doing turnkey assembly for you would get boards fabricated as part of the process.  The the same company is offering the turnkey assembly service that is also a PCB house, then they will produce the bare boards internally.  However, there are plenty of assembly houses that aren't also PCB fabs.  In that case for a turnkey job, they will subcontract out the PCB fab, usually to a company they have special business relationship with just for that purpose.