The terminals can be chosen depending on the need and requirement. It is just an illustration. You can do the other way round as well.

The input side is taken as screw terminal because the input side generally remains fixed(for a power supply it is always the rectified AC mains or may be a battery of higher voltage).

The output side(load) may be changed from time to time(For example load may be a microphone, a mobile phone, an amplifier, etc) so a generic connector is used as the user may change the load as and when required.

Please let me know if any more doubts.

Thank You.

Regards,
Sumanto Kar

This is from OpenPLC subect. A madam who address our doubts suggested me to move to this site to download the hardware and gerber files. How to download them I am not understanding because a bunch of codes are present there which I may copy  but what to do further. I made a sample.hex file in my PC which is a ladder diagram program. Madam had uploaded the same code in the Actual OpenPLC mainboard using USBasp connector, but I don't have the hardware with me right now so she suggested to go to the above website to download the hardware and gerber files. I am not getting anything what to do neither I am finding a way to download the files nor I am understanding how will I flash the code in the simulated software(say virtual hardware), also how to perform the experiments there. You are teaching eSim but this is a part of OpenPLC but I think you can guide me and help me as well. Hope you will address me the answer.