The world's largest web properties serve thousands or even millions of simultaneous users. This is made possible by providing a uniform service to all users with any personal properties factored out of the software and saved elsewhere. This is called a "multi-tenant" architecture by analogy with high rise apartment towers.
High service volume is made possible by computers with the sole purpose of balancing the load between fleets of other computers doing the work that the site offers. So called "application" servers can come and go as load demands and their own limited reliability keeps them in service. Similarly a fleet of database servers redundantly hold all user variable content redundantly so that they might come and go too.
Often small application variations are deployed together with details of user behavior for each version tracked in what is known as A/B User testing. The goal is to find the best "performing" application with respect to the owner's business objectives.
A/B testing has lead Apple's pages to be clean and beautiful like a jewelry store and Amazon's pages to be crowded with many variations of commodity items like a supermarket.
We might think of A/B testing as users shaping their own experience as they move through an evolutionary landscape. But the survival value of each choice is that of the business, its founders and stockholders. Of course the choices are too numerous to be tended by humans so the now notorious "algorithm" steps in.
See Surveillance Capitalism for this situation described in economic terms.