Robots are systems of sensors, functions and motors. To make the robot, you name the sensors and motors as if they were variables in a computer program. Your robot is any program you can make with sensors and motor variables. You build on paper before you build on hardware.
Given sensors SL and SR and motors ML and MR
Now Ml = f(SL,SR) and MR = q(SR,SL)
Designing a robot is defining these functions to let the robot do some action.
Imagine a rectangular platform with ML and MR on the back corners and SL and SR on the front corners. This causes motion.
If ML > MR the robot turns right.
If ML = MR the robot stops
If MR > ML the robot turns left
Now design is to produce the functions that feed data to ML and MR.
Let ML = SL - SR ;
Now ML depends on the difference of SL and SR.
If SL > SR ML runs forward
If SL = SR ML stops
IF SR > SL ML runs backward
Now look at MR
Make MR = -ML and you have a turning system
You make all this on paper, with truth tables before you start on the hardware.
To actually build this requires motor controller hardware, sensor hardware, microcontrollers for functions and a power supply.
Once you have made a robot, you can reuse the parts and you have an infrastructure so it becomes easier as you go. There are robot parts out there you can buy including sensors, motors and functions.
However, you can make any robot on paper and it costs nothing.