> Which ones exactly and how did they pull off this apparent massive conspiracy?

Most of those details are in the findings of the Financial Crisis Inquiry Commission [1]:

"More than 30 years of deregulation and reliance on self-regulation by financial institutions, championed by former Federal Reserve chairman Alan Greenspan and others, supported by successive administrations and Congresses, and actively pushed by the powerful financial industry at every turn, had stripped away key safeguards, which could have helped avoid catastrophe. This approach had opened up gaps in oversight of critical areas with trillions of dollars at risk, such as the shadow banking system and over-the-counter derivatives markets. In addition, the government permitted financial firms to pick their preferred regulators in what became a race to the weakest supervisor."

...

"In the years leading up to the crisis, too many financial institutions, as well as too many households, borrowed to the hilt. ... [A]s of 2007, the leverage ratios [of the five major investment banks] were as high as 40 to 1, meaning for every $40 in assets, there was only $1 in capital to cover losses. Less than a 3% drop in asset values could wipe out a firm. To make matters worse, much of their borrowing was short-term, in the overnight market—meaning the borrowing had to be renewed each and every day. ... And the leverage was often hidden—in derivatives positions, in off-balance-sheet entities, and through "window dressing" of financial reports available to the investing public. ... The heavy debt taken on by some financial institutions was exacerbated by the risky assets they were acquiring with that debt. As the mortgage and real estate markets churned out riskier and riskier loans and securities, many financial institutions loaded up on them."

...

> Near as I can tell, Greenspan pumping trillions of near free credit into the economy is nearly if not more culpable than the loose underwriting at these banks - after all, if they didn’t loosen them, they’d lose their market share in near free credit environments.

None of the firms told Greenspan to stop. They pushed for more and more deregulations and continued to take the money.

> And what about the borrowers that were signing up for these completely unrealistic loans? How do they fit in?

> It’s easy to blame amorphous entities, it’s hard to really nail down what was actually happening - and point out that everyone in the US was playing a part in a spiraling out of control mess.

No, it's pretty well documented. Calling the financial industry "amorphous entities" is... well, just kind of humorous. It's your choice whether to hold the industry that spent decades eliminating regulations for themselves accountable, or the people the industry took advantage of once they were free of oversight.

In line with your argument, if our financial elite don't know more about finance than everyday citizens (why else hold them equally accountable), and they're too incompetent to regulate themselves (else they would have corrected the Fed's behavior), what is their purpose? Wouldn't it be cheaper for everyone if we gave practically interest free money directly to businesses that produce actual goods and services?

[1] https://en.wikipedia.org/wiki/Financial_Crisis_Inquiry_Commi...