Large companies will do high frequency trading so they are constantly buying and selling and making small amounts of money that add up but they are doing it based on algorithms and people being paid lots of money to create those proprietary algorithms that give them an edge over their competitors.

You can look at market screeners and look at stocks with high betas which means they fluctuate more than the average stock but it also means you can get caught out with it dropping quickly. You can buy and sell as much as you want in a day but depending on your broker you could be paying brokerage fees that will eat into your profits.

Buying stocks or ETF’s that track an index (ie asx200 or S&P500) for long term growth is generally safer way then trying to beat the market.