Same way they do it now. Cold applying, word of mouth/referrals, networking events, etc. Personally my first industry job in like 2012 originated from a networking mixer that I showed up to after seeing a promotional online. My unpaid internship/mentorship before that was a word of mouth from my mom asking her friend's husband which I suppose might count as a connection.
But that application goes to a recruiter. Guess if you mean the site shouldn't have recruiters doing the cold-calling, idk maybe, but then the game is in getting the recruiters to read your application. This is different from having connections.
I think OP was referring to applying direct with the company doing the hiring. This is how I landed my first several jobs in tech, no recruiters involved.
Gotcha. Ok the companies themselves reached out on LinkedIn to me. It wasn't those third-party recruiters like "I recruit for Tesla, Apple, Sigma, IBM..." I ignored those emails. But it was still a recruiter within the company, not a technical manager or IC.
Back in the day, CACM use to have job listings at the back. There was always DE Shaw advertising. Sending in printed CVs and cover letters was the slow and painful way to do it.