An excerpt from the writings of Galileo Pilgrim, The History of the Reconstruction, Chapter 2:

As it had since the dawn of the Industrial Revolution, technology continued to progress rapidly. Silicon based computers continued to be optimized and improved upon, until the world had enough computing power for even the most advanced tasks in massive data centers. For a price, complex simulations with precision even to the atomic level were available. Cloud computing was massively pervasive, except in the rare case where an application was made to be distributed - as was the case with the First Reconstructionists application (see chapter 4). But everything eventually meets its end--even the AI Revolution. Eventually, the explosive growth of new technology in computers and artificial intelligence died down to glowing embers, and new gains became marginal. The world grew complacent as AI took over many industries and general wealth grew.
Although a universal basic per-capita income only existed in certain countries, food and shelter were easy enough to get with monetary compensation from small amounts of work. Meanwhile, the most ambitious continued working in scientific fields of many kinds, despite the popular opinion that social sciences were useless, and physical and theoretical sciences were frivolous. The fields, and most sub-fields of science, technology, engineering and mathematics had been progressed to their practical limits, creating unified theories in physics, mathematical generalities, and optimizations in physical sciences. Broadly speaking, these discoveries were then applied to create more efficient, more productive systems in the world. These systems were ultimately used by their owners to generate wealth.
Owners of the most successful types of software grew extremely wealthy, aided by the more advanced forms of patent protection which came alongside the technological or AI revolution. Optimization technology, used to maximize the efficiency of all sorts of productive processes, was both intensely sought after and immensely profitable. Everyone who had a successful career in fields associated with such technology became very wealthy due to their work. At the same time, the welfare systems in most countries allowed those who didn’t want to work such technologically oriented jobs, or sometimes any jobs at all, to still lead quite comfortable lives. Of course, the service industry also provided profitable opportunities, but the stigma around such jobs had grown to a point that caused many people to avoid them.
A sort of de facto class system emerged as a by-product of this progress. At the very top, there were the owners of tech businesses and corporations. The various stages below them were occupied by  the workers in the same profitable tech businesses. A smallish class of hands-on engineering and maintenance workers who supported the hardware of various systems came after the tech workers. Next, there were service industry workers in positions that AI couldn’t replace; and they had a whole class system to themselves based on which group of people they served.
And finally, broadly speaking, there was everyone else. This last group was at least twice as big as all the other groups combined in terms of population, but hardly had any sway in politics or policies in spite of the democratic systems in place in most countries. Until the time of the First Reconstructionists, very few of these people seemed to care for such things. They simply lived off redistribution of wealth, and some of them worked menial, part-time jobs.
The government itself had also evolved with the times. While public institutions still maintained the appearance of control, much of the practical implementation of government was handled by private organizations. Large tech corporations implemented the functionality of the government through the specializations of their AI, performing much of the work formerly handled by public servants. In some cases, these organizations even created policy, since their control often spanned the entire field of their expertise due to their contracts with various governments. While the borders and sovereignty of countries and states remained relatively unchanged, it was common for a single corporation to implement policy globally, changing parameters as needed according to the country. Importantly, this was the case with much of the police system.
Interestingly, the physical vertical positioning of the workplaces of the government, as well as the private sector, is a mirror of the power and class position that each held. During these times, many of the larger cities in the world built a second level in which the higher classes of people lived and worked, above the buildings and streets of the city below. The residents of these places were all individuals of immense wealth - generally the owners and high-level workers in large tech corporations. Skyscrapers, as well as other towering physical support structures, were used to create these new cities in the sky. Governmental offices were typically located in the tops of these skyscrapers - below the upper level of the city, yet above the general populace.
It is unclear which city was the first to create this type of structure, but at the time of this writing, historians have identified 23 locations that had this sort of system. The standard of living of the inhabitants living in the upper level was superior to anything that has been found in the histories of the world (see chapter 3). At some point, these people started to be called the Overworlders, and those who lived in the old cities below were called the Underworlders - likely only by those outside of their respective classes.