So many words for nothing.
Exactly: most of the many words you typed in this forum, were to ask things already clearly explained in the manual.
OK, did I get it right? Number of passengers and extra fuel?
It seems you missed the VGDS integration on that pge, which shows several informations about the flight on those VGDS that supports that ( those with enough lines of text ).
(the fuel truck asks for entry of the main amount into the FMC)
Now it seems you missed the sentence in that same page which says "(provided the airplane has the “Show MSFS “Fuel and Cargo”)", meaning it's not used when the GSX is NOT refueling the airplane.
I think that could be done more efficiently by GSX asking these two data.
I don't know what do you mean with this. GSX IS asking all this ( not just "two" ) data and it uses it.
But your question about refueling seems you now have missed the whole concept ( obviously explained in the manual as well ) on how different refueling works if the airplane has the "Show MSFS “Fuel and Cargo” option Enabled or not, which means GSX IS refueling the airplane or it's not because, as also explained in the manual, some 3rd party airplane have custom fuel system that CANNOT be controlled by GSX, making reading the fuel quantity from Simbrief useless in this case, since we wouldn't do anything with it, if the airplane is refueling itself.