How do early geographers and mapmakers even know what earth looked like prior to the 20th century?

Before photography, advance technology, rockets, satellites, and such took place? This thought somewhat occur to me as I’ve been reading about several histories related to trades, explorations, and expeditions featuring some early maps related to those events. Some mapmakers even came relatively close to being accurate about the depiction of earth. Even during times when people thought the world was flat. For example, the map of Christopher Columbus’ expedition and exploration. How would early geographers and mapmakers around that point know how to depict continents, like North America and such, which no one from their community has ever visited before, or without physically leaving the planet during that time?

(Yeah, I know Columbus was never the first to discover The Americas. That one is just an elaboration I want to pull to address my point further.)

Another notable example is how the maps made to locate Terra Australis, a proposed continent that’s akin to tracking down El Dorado, were designed in the 16th century.

Cartography seems like an interesting subject and I don’t really know too much of the field, unless I could gain more elaboration from this.