Confusion about why voltage of a cell decreases as it's used up and how it relates to current
If you load up a motor with a physical force the motor starts to decrease speed as the torque from the physical load ramps up. Eventually the motor slows down so much it stalls and you are left with zero speed and maximum torque.

It's the same for a battery. Without a load it runs at full speed (open circuit voltage) and as you load it up the terminal voltage lowers as the current taken increases. Eventually, with a shorted out battery the current taken is at maximum but the terminal voltage is zero.

The internal resistance of the cell causes this to happen. If a cell didn't have internal resistance it could supply any amount of current without the terminal voltage falling (an impossibility of course).