I have literally never heard anyone say that in my life.

Its a common phrase in the UK.

“The boys” in this context are a managers cronies, not any random person who happens to be male. In fact the phrase excludes men too, who don’t play golf or whatever.

I would say its a bit more general than that. Family members getting jobs via a parent would also be labelled this way.

Agreed. A manager hiring their daughter over more qualified candidates would be a “jobs for the boys” situation despite her not being a literal boy.