Back in the fall of 1984, when I started my first official semester as a student at University, my mom was the person in the University HR department that was responsible for running the Job Location Program. That is the program that helped students hook up with potential employers, both on campus and off campus. For temporary jobs and permanent ones.
At that time, she told me that 90% of the placements that she had seen throughout her career in HR, came from people you know, or people that they know.
Of the remainder, 90% (that’s 9% of the total) came from headhunters looking to fill a particular job.
The final bit (just 1% of the total) was done through advertisements and organizational efforts like her Job Location Program.
I don’t think the numbers have gotten any better since. If anything, they’re worse.
So, it’s always been mostly “who you know”, from hiring, through working, and then your exit.