Where does the US Dollar go when you buy a bitcoin? Not From Anyone but from CoinBase ect.
When you buy a book from Amazon (directly, not from its marketplace), who gets the money?  Amazon gets it.  They can use the money to buy more books to sell to other customers, pay their employees, power their servers, etc.

When you buy Bitcoins from Coinbase, who gets the money?  Coinbase gets it.  They can use the money to buy more Bitcoins to sell to other customers, pay their employees, power their servers, etc.