We used a five-star-based rating system to rate companies in the discount stock brokers list above. The best brokerage firms would get the highest, five-star rating. In 2020 not a single firm got five stars, however six brokers were rated at four and half stars. Any brokerage house with two- or one-star rating should be avoided. Three-star rated firm is perfectly fine, but there are, probably, better options for investors to consider. All companies with three and half stars and higher are recommended for at least one category of investors. 

In the professional world, one of the key concepts is diversification. Harry Markowitz is a Nobel prize winning economist and one of his major discoveries was that adding new asset classes can dramatically alter the overall risk profile of a portfolio. His finding was that a portfolio that contained very low risk assets would normally benefit from lower volatility and higher returns if a higher risk asset was added. This is due to the likely lack of correlation between high and low risk asset classes.
For online platforms, websites give the user options and availability of the stock he wishes for, across all the platforms available. If he places an order, it gets stored in a database. After the confirmation of the trading account and payments, the order gets forwarded and the user gets his stock, in the form of money, transferred to his trading account.