I had a work mate that used to be a chemist at the Anacortes refinery in Washington state. He told me that the trucks would fill up and then, based on their company brand, get the additive shot they needed. i.e. Chevron and mobile and Arco were all the same gasoline, but each got a different additive when the truck was filled.
Think of the addition of color to a base paint at the hardware store.
This was close to 20 years ago and before ethanol was common in gas.
I can see the Anacortes refineries out my window as I am typing this and yes, that is how they do it. I’m lucky enough to have two stations within driving distance where I can buy ethanol-free gas. Costs a little more but my old weed-eater and leaf blower that leak oil like a sieve and no longer run remind me what that crap does to a motor.