It is investors who predict that the stock price will rise, but their own funds are limited, so they can't buy a lot of stocks. Therefore, they pay a part of the deposit first, raise money from the bank through brokers to buy stocks, and then sell them when the stock price rises to a certain price, so as to obtain the difference income.

Short selling means that investors predict that the stock price will fall, so they pay mortgage loans to brokers and borrow shares to sell first. When the stock price falls to a certain price, buy the stock, and then return the borrowed stock to get the difference income.

I've never heard of buying more books, most of which are called reading more in the market. Generally speaking, it is optimistic about individual stocks and buying.