Ex coworkers. Last time I got a job traditionally and walked in cold to a job where I didn't know anyone there was in 1994 or 1995 don't remember exactly. Ever since then its all been "Yeah (fill in the blank) who worked with you at (fill in the blank) mentioned your name and the work you did with (fill in the blank) so I figured I'd give you a call and ..."
I predict I'd be freaked out to walk into a new job and not know anyone. I haven't done that in twenty years. I think I'd be lonely for awhile? I have some respect for people who do that, its gotta be very stressful.
Even noobs can kind of apply this via school classmates and to a much more limited extent physical meatspace users group meetings. Locally a makerspace is slowly bootstrapping itself and once its further along, I'll join and meet some interesting people soon (the pioneers of the group only meet while I'm busy at work, so occasional mailing list interaction is possible, although not in person).
One hardware job interview a long time ago involved pulling circuit boards of stuff I designed and built at home out of my suit pockets during the interview. The HR lady was stunned, and I was also stunned to hear that she was stunned. Admittedly I was bringing analog to a digital gunfight but still... they're looking for a hardware dude, and I got interviewed because I know a guy, but none of the guys you selected by yourselves personally own a soldering iron, and you're hiring for a HARDWARE guy? If HR screws up that badly, which happens often enough, if you can find any way at all to get in, you'll basically be the only candidate. There are probably ways for HR to do this with software, maybe HR ends up only providing java candidates but they actually need a Ruby guy and you're a Ruby guy and via an old friend you get interviewed, well, I guess you win.