ELI5: How do land surveyors tools work?

I understand what a surveyor accomplishes, but am wondering exactly how the tools work. GPS these days is only so accurate, assuming they still have to at least verify with older ways (and what I'm more curious about). Take a theodolite for example, I understand it gives surveyors the ability to triangulate a position with Horz/Vert positions, how? Especially in areas with trees and no "markers" to allow them to have an accurate starting position. Currently selling my parents house in the country. There are no manholes or street markers nearby with a definitive location that I can find (walked the road many times). The borders are covered with tree's so no real line of sight aside from the road.

self