the owner of the power plant hires a contractor to perform the outage inspection of a piece of equipment. Say for a turbine, there will be the need for varoius crafts and services. this will include milwrights, riggers, I&C technitions, sand blasters, NDE, bladers, welders, machinest, winders, foremen, supervisors, technical directors and engineers.
If the owner wishes, they can have seperate contracts with varoius companys to perform the outage OR, hire one company and let them be reponsible for providing all services. Basically the owner "gives the keys" to this contractor and when finished, the contractor "returns the keys". vary rarely will a true "turn key" outage occur, since that would require the contractor to perform the remove from service, tagout, clearing, and return to service activities.
I have been on so called turn key jobs where the contractor had to proviide thier own house power and facilities