These workers are part of building & construction trades, members of organized craft unions. Craft unionization can trace itself back to medieval times, when guilds were formed to represent various skilled professions. Knowledge was passed on through apprenticeship programs, so that young workers gained the necessary skills.
Construction trades workers were some of the first to organize in the United States, beginning at a city level in the 1830s. By the 1880s and 1890s many national craft unions were formed under the auspices of the American Federation of Labor. The construction trade unions are today the longest lasting heirs of this tradition. Carpenters, Electricians, Plumbers, Pipefitters, Bricklayers, Laborers, Cement Masons and numerous other trades have specific skills sets appropriate to their trade. They continue to maintain labor-management funded and supervised apprenticeship programs to pass on their knowledge to younger workers.
Illlinois can claim the foundation of many of these trades. Iron Workers Local 1 is a Chicago local; Sheet Metal Workers Local 1 is in Peoria. The Laborers first national gathering was dominated by Chicago workers. Many building trade craft unions in Illinois are nearing or over a century old and continue active today.