Stock market

While I was studying I came across a word SHORTING and the explanation I found was "If A person buys a share from stock/bond broker for example 100rs and sells the stock in open market for same 100rs and when the stock price falls for like 80rs buys back for 80rs and sells them for 100rs to the stock/bond broker making their profit of 20rs" I think the stock/bond brokers are like zerodha,angel one,upstox i don't know what is open market please tell me and I have an another doubt if the stock price is down to 80 how can he sell it to 100rs to the broker. PLEASE HELP ME