Once the board has paste applied, the components are placed on top. This can be done by hand or by a machine. Using tweezers is a great way to place components. There is a common fallacy that you need large, expensive machines to make electronics. Not true! A human is quite fast at placing components and the surface tension of liquid metal is such that most components will shift into their correct location during reflow. But humans have a limit to their stamina. http://www.etonautosmtmachine.com/led-pick-and-place-machine/