They can use what I usually use for building fortifications. “Modular” construction. I have templates for a piece of straight wall and corners, then stamp them next to each other and build walls as long as I want them, one part at a time.
Simple structures like bridges can be done in a similar fashion. I don’t think a separate “tool” for bridges is required - firstly, drawing a primitive bridge is as simple as drawing a rectangle, and secondly, it is not much of a “geometrical primitive”, as with walls or slabs. I personally would like to encourage players to experiment with the builder here. Bridges are not as vital as buildings for gameplay, at least now