So apparently it's watts divided by volts equals amps.
So
7300w÷12v=608amps for the semi
and
2500w÷12v=208amps for the pickup.
It's easiest to explain these terms by comparing it to something we can actually see and visualize.
Water
The capacity of a battery is measured in amp hours. For example a 100 amp load for one hour is 100 amp hours.
Car & truck batteries are rated in cold cranking amps which is how many amps a fully charged battery can deliver when it's like 5 degrees. It'll be higher than that in the summer.
Anyway pretend amps is unpressurized water, measured by volume.
Volts would be the pressure. You'll move more water through a hose if you put some pressure behind it.
Watts is a measurement of the total water flow coming out of the hose. With more pressure, (volts) you'll get more water through the hose, however a fatter hose will deliver the same flow rate with less pressure.
That's pretty much where the water comparison ends.
A battery's current state of charge can't be determined as easily as we can measure the amount of water in a tank.
So the scientists have worked out a system for determining state of charge based on the voltage reading.
It's kind of like trying to estimate how much urine is in your bladder based only on how much pressure you feel. Obviously you can just piss in a measuring cup and determine it very accurately, but at that point the bladder is empty and your measurement is how much WAS in there, not how much is in there NOW.
You'd have to fully discharge a battery & record how long it powers a light bulb or something in order to know it's current state of charge at the beginning of the test, but then the battery is totally dead afterwards.
So measuring the voltage of the unladen battery is all we can do. But the scientists have worked out the relationship between voltage and current state of charge, and provided us with charts to use.