I'd just like to hijack this thread to post one of my all time favourite websites:

https://maps.nls.uk/geo/explore/side-by-side/swipe/

It's the National Library of Scotland's map site's side-by-side viewer which lets you browse classic maps synced with modern ones. UK-centric, but it still has loads of global and international maps too.

Lots of replies about triangulation etc but the basic concept if a top down map was a military development by - Gallileo. That guy did so much

Might I suggest this article about French mapmaking in the 1600s. Supposedly in response to a map commissioned by King Louis XIV at the time, which shower France about 20% smaller than originally mapped, the King replied:

"Your work has cost me a large part of my state!"

At first, people used itinirary (to go to Rome, first go to PlaceX, then PlaceY, etc)

Maps of the time, when they existed, where merely illustrations, with fictionnal places sprinkled in.

Then some attempts mesured distances between places by counting step, and overall direction. This is why most maps before 1000ce are really strange looking, but you can guess what they represent using the name of places.

With the invention of things like the astrolab, people where able to mesure both distances and more precise direction, giving birth to triangulation, which was used until the invention of GPS

A lot of others have already mentioned the use of trigonometry. I would like to add that, often times, the position of the Sun, stars, etc. were used for navigation. I would imagine their positions would be useful to judge large distances. And many times, map makers asked those living in the area about regions they themselves have not explored in great detail. Lots of maps are also an amalgamation of many other maps, so although it is unlikely one person explored vast swaths of forest by themselves, it is more likely that different parts of it came from different people, including locals.

p.s. Eratosthenes, an ancient Greek polymath from 200 BCE ish (!!), measured the circumference of the Earth to a remarkable accuracy simply by using shadows of a vertical rod cast by the Sun. It's kind of insane how people were able to do all this without the assistance of modern technology.

Everything was/is mapped with trigonometry. You map the big triangles first, then once you have some nice big markers at known distances in the hills around you you can just measure angles to work out where you are. Repeat that lots and you can record the exact location of everything.

Your question is not stupid, but it is overly vague and broad. An equally vague and broad answer would be: "They did their best with the tools and knowledge they had, tried to copy other cartographer's work (and failing that, their techniques), and otherwise just guessed. Failingthat, they made it up in ways that would benefit themselves the most.

Source: I read the Map Men's book not too long ago.

step 1. you take a piece of paper
step 2. you paint a picture of what you think the landscape looks like from above
step 3. you add names and descriptions to all the small icons that you just drawed