You've never worked on a survey crew. The map makers probably used the direct distance measuremnts done by the surveyors, actual measurement of distance from place to place, something that has been doen since long before the spherical concept of the earth was developed (Roman maps and roads from 2000+ years ago are quite good). Basically, you just set yourself on a heading and go in a straight line in that direction and you arrive at your destination (grravity compensates for the earth's curvature so that you don't end up out in the vacuum somewhere on a long journey. You don't even have to think about it, it's all automatic. Same way you don't have to remember to breathe, it just happens).
You are corect, I have not worked on a survey crew.
Not if you measure the heading from a map. In fact a great circle route, which is the shortest distance between two points on a shere, does not maintain a constant heading, relative to true north. Except in the special case of going due north or south.
Get a large shere, a basketball or a large beach ball, and experiment, remembering that north is the direction towards the north pole. Maps show north "up" at both San Diego and Portland Maine, but in 3 dimensions on the surface of the earth, norht is not the same direction at both locations. In fact if you are at the equator and want to go to anther point on the equator on the opposite side of a sphere, you'll find that any of the cardinal directions, (NEWS) will get you there, and at the same distance. This is not true for the earth, because it's not a perfect sphere. GPS uses a system which assumes its an ellipsoid of revolution, like a slighty squashed shere, and that's pretty close, but not exact either, as the earth is actually a bit pear shaped.