Address the impact of class structure: how many different classes are evident in To Kill a Mockingbird? Think beyond the obvious of black and white. Would this structure and the reaction of each component in it only apply to small towns in rural areas of the country?

At the time of the setting of Harper Lee's novel, the 1930s, Alabama was a state much like other states in the Deep South. Most of the people were descendants of families that had come to America from Great Britain, some migrating from Virginia to the Black Belt, and others from the Tennessee Valley or North Carolina. The southern part of the state differs from the others because the French and Spanish explorers came to Mobile and other coastal areas much earlier.
A state constitution was passed by Democrats in 1901 that disenfranchised most African Americans, who comprised more than 45% of the state's population at that time (this was before the Great Migration). Poor whites did not fare particularly well, either. They were often sharecroppers, and at the end of the harvest, they had very little to show for all their work. (Mr. Cunningham exemplifies the sharecropper.) The least ambitious of the poor whites were those referred to as "white trash." (The Ewells exemplify this group). They had a faded-out lack of color to them, and as Scout observes, many of them seem to resemble one another, perhaps because of poor nutrition or inbreeding.
Since the growth of industry did not occur in the South as it did in the North, there was not the same influx of various nationalities and people from other countries who sought work in the mills, factories, and shops of the North. Even some small towns in the North had factories (often those located near rivers because waste products were expelled into them), so different people came to them. For instance, many Polish and Italian immigrants moved where there were steel mills in which they could work. In Alabama, there are many descendants of Italian immigrants who reside in the Birmingham area where there were steel mills (this city was called the "Pittsburgh of the South"). Throughout the rest of the state, however, the names that were in the phone books in this north-central area were predominantly names from the British Isles.
As Scout narrates, the small towns of Alabama "grew inward" because there was no industry to draw new people. This is why Aunt Alexandra categorizes the families in Maycomb in chapter 13 as having certain genetic "streaks" in them. Other rural areas in the country are similar in that particular surnames prevail, but to this day, the South does have a concentration of people whose ancestors came to America from the British Isles long before those who entered Ellis Island. For this reason, there is a somewhat definitive culture in the Deep South. Such concentrations of one nationality are usually only found in certain neighborhoods of major cities, certain suburbs of those cities in the North, or in specific geographic areas of a state.
https://www.history.com/topics/us-states/alabama