Do you know what simulation software does? Math. Just math.
In my engineering classes, we had to go through reams of calculations and free-body diagrams and such, and I asked why all of this wasn’t just programmed into design software. My professor responded that it absolutely was, but we kinda felt like actual engineers should know what the computers were doing, if we were going to rely on them to protect peoples’ lives.
The point is that even ancient people figured out mathematical principles behind design, and did at least some type of calculation of what would be required.
A lot of this was assisted by the use of scale models, and the math involved was primarily about understanding how it would scale up.
An example, which isn’t ancient, but is certainly pre-computer, is when Antoni Gaudi was designing the Sagrada Familia in the 19th century. He designed the incredibly complex system of arches that made up the frame of the building, in part by realizing that the arches made by hanging weights from a string or chain were the inverse of the ideal arch needed to support a weight on top. Accordingly, he build upside-down models of the structure, using hanging chains and weights.
Point is, human ingenuity and mathematical ability weren’t born when computers were invented. Computers allow us more precision in our designs, make our buildings safer under wide-ranging circumstances, and are certainly much, much faster than doing calculations by hand. But the ability to design a functional structure has manifestly been around for a very long time.