Just my experience, it may not be typical, but I was getting a replacement single story extension on my old place, and moving a few rooms around, which included putting the new kitchen into what had previously been a downstairs bathroom: so it was all new in that sense.
I had trouble finding a builder who was free and willing to do the work, they always seem so busy that they only want to cherry pick jobs. Finally got one. He suggested I use his discount to select kitchen units for the new kitchen on top of the building work I had been quoted for; and have him do the work for that too. which I agreed to. He chose the other tradesmen, plumbers, electrician, etc.
I wasn't that impressed. Afterwards I had to rip out a lot of the electrics as it was hopeless. Sockets that would never be openable as they were tiled and grouted into place between the tiles. I couldn't find the cooker switch, all were mains sockets on the wall, until I looked under the units to find it wired and loose on the floor. Too much to detail here, and I still do not know why I missed it on the final inspection. It took me a year of weekends (I had other things to do) to correct the idiocies.
IMO employ your own tradesmen to do whatever the initial job was, and don't trust them to call in their incompetent mates to knock together some bodge that wasn't part of you initial enquiry with them.