let's say i want to make a medieval style city with road and a "sidewalk"

if i want to have the buildings built into the sidewalk then i can just implement the sidewalk as a 3d object

but if i want there to be a combination of grass and sidewalk then i need to use the terrain editor

the problem is i can't figure out how to make a 90 degree angle using the editor. or if it's possible to press a button so i can keep the terrain height coordinate constant as i "paint" the area of lower elevation where the road will be

A heightmap is not precise enough for that kind of work. I'm thinking your best bet would be to make some 3D sidewalk objects and add them as terrain objects. Depending on how it's done, you could possibly also get away with a 2D object(plane).

you can do it with heightmap but then you would have to really construct the sidewalks etc around the terrain instead of molding the terrain to your town/city layout.