There is a microcontroller in ROSbot, it is a STM32 device, it is placed on a CORE2 board.
ROSbots are sold with Asus TInkerBoard or Aaeon UpBoard, depending on version. Both devices are single board computers like RPi and in the same form factor, but differ in performance.
Rosserial is connecting SBC and microcontroller in both directions.
The SBC sends velocity command to micrcontroller via rosserial.
The microcontroller sends odometry and sensor data to SBC via rosserial.
User is interacting with SBC only.