just in case you haven't already seen it, David Rumsey's website is the most amazing collection of historical maps. https://www.davidrumsey.com/

they usually used triangulation, the same as today. they select an line as the startimg point and measure it exactly as possible(doesn't need to be long) and then you can calculate the rest with the measured angles of the area you want to measure. The rest is replicating it on paper with the required factor.

You might find the retriangulation of Britain in the 1930s and 40s interesting [1]. A network of ~6,500 triangulation pillars (trig points) were built across the country, each of which were in line of sight of at least two other pillars. The pillars each have a mounting point for a theodolite, to allow their position to be calculated from the relative angle and elevation of other visible trig points.

They are often on the top of hills or mountains and so have a warm place in my heart from signifying the high-point of many a hike.

[1] https://en.wikipedia.org/wiki/Retriangulation_of_Great_Brita...