Where are these paid links coming from?

It's been over 10 years since I've done any kind of SEO work. Back then there were networks like Text Link Ads were you could buy/sell links. Do those still exist? I did some searching but it looks like now there's all these agencies that build links for you. I was doing some backlink research and came across a site with a bunch of posts that contains links which are clearly paid. Here's an example LINK\_HERE The interesting part to me is that the site actually looks like a legit company offering services LINK\_HERE, so I wouldn't expect to see them selling links like that in their blog posts. Are the link buyers all reaching out to this site (and others) individually to buy links? Or is it more likely that they work with a single agency who buys all those links? Or is there some secret link exchange network?