GCWR and Tongue Weight

I would like to know if the GCWR is truly the ultimate weight that is the combined Curb weight of the truck + the loaded weight of the trailer?

A GCWR number is a rating number. Truck curb weight + loaded trailer weight is a GCW number.
The truck makers GCWR can be any weight he chooses.
GCWR is not on a sticker or placarded on your truck anywhere and has no legal usage.

Or is the factor that matters solely the tongue weight not exceeding the GVWR?

Tongue weight rest on the trucks rear axle/tires (RAWR). That is the factor on any truck for how much weight it can carry.
GVWR is another number that isn't used in any legal sense for how much load your truck can carry. However some folks use the GVWR for how much load on the axle/tires their truck can carry. RAWR or GVWR are both certified and both can be used for figuring payloads.

I've seen so many trucks seemingly hauling massive loads and yet those number contradict the manufacturers weight ratings.

Any help to educate me on the topic is appreciated.

I don't know what you saw nor could I guess at any weights just by looking but trucks carry weight determined by the axle/tire load rating. Some states may use a GVWR for tax purposes or for registering for a tag/etc. However my state as others have no weight registration. In the case of a GN/5th wheel trailer we can carry weight up to the top of the RAWR/tire ratings.
I generally tell new folks to the towing world to talk with your state motor vehicle size and weight officer. They can tell you which numbers are used.