They're nicknamed 'tacos de sudor' — sweaty tacos — because they're steamed and stacked inside a cloth-lined basket, where trapped heat and condensation slowly soften the tortillas over hours of selling.
Small tortillas are filled with simple fillings like refried beans, potato, or chicharrón, then stacked tightly in a basket and lightly steamed as the vendor pedals or walks a fixed route. Vendors around Mexico City sell them from bicycles and handcarts, usually before mid-morning, splashed with salsa and pickled vegetables to order.