In modern professional sand sculpting, much of the art is rendered by reductive sculpting, similar in method to the way "hard" mediums of wood or stone are "carved."
But modeling or 'add-on' sculpting is also done, often together with 'carving,' to create a single artwork.
It is most common that wet sand is compacted, often by machines, in successive layers into rigid formworks of successively smaller sizes.
In this fashion a densely packed block of sand called a "Poundup" is created (see fig. 01)
They're called 'Poundups' because you 'Pound' successive layers of sand into forms from the ground, 'Up.'
Each layer of wet sand that is compacted is called a 'lift.'
Several lifts make one level, or tier (fig.02A & 02B)
This method is most suitable for rendering large and durable 'Blocks' (forms of various shapes) which may not be otherwise attainable nor as long lasting, by simply stacking moist or wet sand by hand.
(The method of 'hand stacking' too however, is indeed completely valid and is widely used in most sand sculpture applications in one form or another, as well often well into the actual sculpting process)
Wooden form work is very commonly used, though plastic, metal, cloth and other materials may be used. Wood frame forms of various shapes & sizes are built by attaching interlocking pieces in a way that can withstand machine compacting of the material, but also be removed quickly & easily later. (fig 03)
After a poundup is complete, sculpting can begin. This is done by removing the formwork from the top down to expose the dense block of compacted sand for rendering. (fig. 04 & 05)
Formwork for poundups can be in myriad shapes and configurations, using wood forms, round plastic ones, or a combination of both. (fig 06)
As noted, the poundups are made from the bottom up. But the sculpting is done from the top down. So, the actual sculpture is generally created to a finished state from the top down. This is different than with many other mediums that can be worked from all angles and in either direction. Going back to a higher point to rework something can be a challenge (fig 07)
The right machine for the job.
Whacker Brand "Jumping Jack" whacker/packers.
Used for compacting the sand, generally into rigid form work.
I prefer the smaller 720 or 640 Whacker brand models, powered by Honda, as opposed to the bigger diesels by Bomag. There is such a thing as over-compacting which can lead to problems such as 'fracturing' or 'pumping' - but that is a whole other thing.
A single, symmetrical poundup
Layers of sand mixed with water are machined compacted in successive tiers.
Interlocking pieces create wooden frames or form work.
Removing the formwork in layers from the top down as sculpting progresses
An octagonal poundup of a dozen tiers being sculpted to finish from the top down.
Wood forms and round plastic forms can be used together to make myriad configurations
Sometimes if a repair needs to be made, or an artist wishes to change something high up, it may be out of 'easy reach.' In this case, creative solutions are called for.