¿Quién construyó las ruinas — y cómo?
Was it slave labor? Mostly no — and this is one of the most common myths in ancient history. These societies did hold enslaved people (war captives, debt slaves like the Aztec tlacotin), but the great monuments were overwhelmingly built through labor-tax systems: the Inca mit'a (every household owed the state rotating turns of work, paid in food, chicha, and cloth from state storehouses), Mesoamerican tequitl/corvée owed by commoner wards and tribute towns, and Andean communal reciprocity (ayni, mink'a) repaid in feasts. Alongside them worked full-time specialists: architects, stonemasons, sculptors.
With what tools? No wheels for transport, no iron, and no large draft animals (llamas carry only ~30 kg). The toolbox was stone hammers, copper and bronze chisels in the Andes, obsidian blades in Mesoamerica, fiber ropes, log rollers, sledges, ramps, rafts and canoes — and superb organization of human muscle.
How long did it take? It depends what you're building: a Nazca geoglyph took an organized crew days or weeks; a temple enlargement, years; a great pyramid like Teotihuacán's Pyramid of the Sun, decades; a city like Tikal or Chan Chan, centuries — usually built layer upon layer over earlier versions. Machu Picchu occupied crews numbering in the low thousands for a few decades and was never quite finished.
What were the sites for? Almost never just one thing: temples doubled as calendars and political theater, plazas as markets and cosmic stages, “fortresses” like Sacsayhuamán were also sanctuaries, and Machu Picchu was a royal estate. Around every monument stood the real machinery of daily life — terraces, canals, reservoirs, storehouses, and neighborhoods of ordinary builders.