By running a business that puts a lot of pressure on the worker to squeeze them as much as possible, but without actually having any profit margins so that the business runs the competition out of business and keeps growing. The way Bezos is rich is by selling shares in the business to people who expect large profit margins in the future. Whether this ever happens is yet to be seen.
Overall, the shipping part of amazon has been consistently unprofitable but running the marketplace has given them a lot of competitor data when expanding into new niches. The real moneymaker is Amazon web services, which is glorified server hosting. In that sense, the surplus value is counter-intuitively taken from their high-paid workers (mainly software devs) to subsidize the work of the low-paid ones, even though the latter are physically pushed a lot harder.