Board level assembly is typically robotic and inexpensive. I have seen some low cost pick and place machines with arms that move too quickly to track with your eyes. Bottom level pick and place machines start at only a few thousand dollars, although those are a bit slower. So you program your pick and place machines with all of the component locations. Load the parts, and just let things run. The boards go through a solder reflow oven afterward. Then you scrub the boards clean. A few thousand boards can be made quickly this way. Humans make mistakes. If the robotics are less than perfect, the system notifies you and tells you where.