Not sure if you'd consider the company I work for as a big player; however, SEO is definitely one of our most profitable channels and we gross tens of millions a year.
The way we operate for links is all internal. It starts with what I'm targeting for SEO, a content strategist uses their creativity to scope a project to meet my goals, we kick it off with a production team of writers/devs/QA, and then our outreach team leverages existing relationships and cold contacting to do PR for our content or acquire a guest post. After that process, fresh links start rolling into our sites.
However, with that being said, I've used vendors from BHW to jump start a strategy or quickly make up for lost ground with some gray hat links.