What is this breadboard that can be soldered on a PCB
The small one is to connect electronic components in the IO's it has on. That's why it's soldered. You can program the components connected to those pins. The other breadboard is for prototyping. You can wire and connect any sort of electronics you want (sensors, servos, resistors, potentiometers etc.) and then use the IO's of the small breadboard to communicate with them (get sensors' data or "tell" a servo to rotate).