Exactly?

A very smart/savvy person (or an idiot snake oil salesman, who portrays as an expert) has a plan to invest money in a certain way to get larger gains (hopefully) than just playing the market the traditional way.

That person creates a hedge fund, and offers investors to join. Usually this is a private affair, and the HF is a private company itself. So it's invite only, average Joe will not be able to invest with the HF usually.

It's the investors who supply the capital. They bring all the money into the management of the hedge fund. Then the Hedge fund and it's MDs and other employees, they take that money and use the strategy the creator/owner had in mind and they try to make those gains.

HF makes money themselves from their internal capital being invested and seeing gains and profits. HF also makes money of it's investors and their gains, through taking fees for those assets under management.

That's basically it. Technically you can start your own hedge fund if you wanted to, out of your garage. But you'll have to either have millions/billions of your own money to start with, or you'll have to convince millionaire/billionaire types to give you their money to manage. That's the main obstacle that makes it extremely tough to start a HF. Because no one knows you, no one trusts you with that insane wealth.

But when you get to someone famous and trusted, like Madoff.. he was able to scam the whole world for a couple decades because everyone trusted him, he was super respected in the industry. So no one looked at the numbers too closely. They just gave him their money and thought they were making insane profits.

Your loved ones.

I know many parents would immediately sacrifice their own health if it meant the safety and prosperity of their children. Same can be said for your other loved ones too, sometimes!

Also, your legacy. Many people sacrifice their own health or life to affect change and leave their mark on the world. Because that legacy will last way longer than their flesh.