I think it can be 1st or 2nd fix depending on the professional and adult chit chat you've had with the builder before any walls or ceilings go up.
With my builder I tend to have the cable in place in the void for plasterboard walls, then the builder puts the walls up, then I cut my box holes. the builder is decent and he makes sure the cable is not knocked out of position so I just pop my hand in and fish it out.
For downlights it's a similar process. I leave the cables above where they will need to be pulled through, builder puts up the ceiling, I drill the hole and pull them through. I draw a rough plan of the joists so that I don't cut too near them. Again the builder thinks of his fellow tradesman and doesn't dislodge the cables out of position.
In the most recent example, there are too many cables above and I may not be able to find them through a downlight hole, or rather I'll be pulling the wrong cables, so the builder said he'd drill the tiniest hole possible and pull them through. He'll keep it really tiny so that my hole saw arbour can still get a grip.
I have worked with only 2 other builders but both of them were open to chatting about the way forward on the job and not too set in there ways.
Sensible, professional, adult communication whilst thinking ahead for your fellow tradesman be it the builder, plasterer or even....... dare I say it....... the plumber!