The Power Circuitry regulates the high voltage from input parallel jack or battery terminals to the 5V which powers the OLED, Arduino, relays and rotatory Encoder.

External relay Switching circuitry is there so that if you don't have mosfet bank, you can actually use an automotive relay instead of a MOSFET bank. Learn to make a spot welder from an automotive relay from the link below.

There is also a relay for AC switching so with this design and PCB, you will be able to use this as a timer circuit to make a spot welder from Hacked Microwave oven Transformer.

We actually have the Mosfet bank for Battery Spot Welding. Remember, all the power MOSFETs must be of the same batch and of same number with low Rds.

we have a footswitch, and rotatory encoder as the Inputs and we have a 0.96-inch I2c OLED to display all the parameters.

and at the last, the Arduino Pro mini microcontroller processes all the information and actually acts as the brain of the system.

Others are just passive components to support the other electronics. But hey, you don't need to bother with the circuit and PCB as I have already done this for you