The tunnels were built by normal construction workers and methods. Most of the cities were being developed after steam powered earth movers were a thing, and engineers knew how to safely dig under foundations for a long time prior to that.

My city has been destroying parts of them, and blocking off others, as things get demolished and rebuilt. Where I work used to have a usable tunnel that went to an underground speakeasy during prohibition, with era salacious mural walls, when I started. It also connected to a few nearby buildings, still, at the time, and was at the same level as the freight elevator. It has been been blocked off and partially destroyed. That whole basement section predates the current building, above-ground, by several decades. There used to be a whole network, that connected to bus stops (now stairwells to isolated basements, but the bus stop locations haven't changed), and the train station.

Back when pedestrian and horse travel was the norm, rather than everyone having cars, and tractor trailer deliveries being common, the underground was where industry moved their goods, away from retail and commuter traffic. The tunnels weren't clandestine, they were just a boring matter of fact. Now, downtown industrial production in cities is rare as hen's teeth, but that was not so, from the late 1800s up into the 1960s.

Now, mind you, that's just the tunnel systems in the hearts of cities. On the whole, there's still massive amounts of things that don't add up.

The tunnels were built by normal construction workers and methods. Most of the cities were being developed after steam powered earth movers were a thing, and engineers knew how to safely dig under foundations for a long time prior to that.

My city has been destroying parts of them, and blocking off others, as things get demolished and rebuilt. Where I work used to have a usable tunnel that went to an underground speakeasy during prohibition, with era salacious mural walls, when I started. It also connected to a few nearby buildings, still, at the time, and was at the same level as the freight elevator. It has been been blocked off and partially destroyed. That whole basement section predates the current building, above-ground, by several decades. There used to be a whole network, that connected to bus stops (now stairwells to isolated basements, but the bus stop locations haven't changed), and the train station.

Back when pedestrian and horse travel was the norm, rather than everyone having cars, and tractor trailer deliveries being common, the underground was where industry moved their goods, away from retail and commuter traffic. The tunnels weren't clandestine, they were just a boring matter of fact. Now, downtown industrial production in cities is rare as hen's teeth, but that was not so, from the late 1800s up into the 1960s.