Those giant things are literally not pumpkin, they are a variety of squash, ok I suppose pumpkin is too but a different branch.
Growers get seeds from previous winners. Grow in very rich soil, start with several plants as they are male female, get a promising fruit going, eliminate the rest. The garden bed is usually a whole acre. The fruit us grown on a hammock.
There are articles you can Google for a more comprehensive explanation.
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