How does stock ownership of company founders work?

How much control do founders of companies actually have over their stock ownership of the company. For example, Sergey Brin owns 15% (I don't know the exact %) of Google Inc., and let's say he wants to spend a billion on a personal investment. Can he just take that money and transfer it to his personal bank account? Doesn't the stock money exist for growth purposes and for further development of a company - after all, sharehlders expect a return. Please enlighten me. Knowledgeable answers only please. Thanks a lot in advance!

He owns the stock and is free to sell $1 billion worth if he'd like. The money raised from OUTSIDE investors through the IPO is intended to help the company. For founders it's a liquidity event that gives them a market for the stock they own.

Here's the quick and dirty answer.

Brin has the right to sell his shares to anyone he chooses. He might sell them to the company, or he might dump them on the open market.

Anytime he buys a share, sells a share, writes a stock option, or borrows against Google, be it 1, 100, 1000, or 1,000,000 shares, he is required to file a disclosure with the Securities and Exchange Commission of an "Insider Transaction". This immediately becomes public record.

As a major stockholder, he is probably on the Board of Directors. As such, he has a fiduciary duty to act in the best interests of his company in his decision making or risk a lawsuit by other shareholders. Dumping 15% of the company's stock on the open market would put the stock in a nosedive. So he has a duty to act in a more responsible manner than that.