You are just buying shares that someone else is selling.

That's why it's called a Stock Exchange.

The only time the company gets the money is when there is a new stock issue, called an IPO or something similar.

New stock issues are usually handled by a broker, underwriter or investment banker, and then listed on a stock exchange like NYSE or NASDAQ for subsequent active trading.

The investment banker sells the stock to investors/shareholders, issues stock certificates thru a registrar, and gives the issuing company the net proceeds after deducting commissions and underwriting fees.

Then, active trading can begin on the listing stock exchange.