Bearing balls are commonly machined to unbelievably tight tolerances, like, deep submicron roundness, I think. This is exploited in many kinds of precision machinery. They’re also quite hard and cheap.
So if you take two bearing balls that are coated in some sort of sticky liquid or plastic material such as clay, and you press them together until their surfaces touch, their centers are a very precise distance apart. If the sticky substance then hardens without expanding, it will preserve this very precise distance.
If you have three bearing balls thus all mutually attached, they will have not only precise distances between them, but also precise angles. If you add a fourth in contact with the first three, it will be in a precisely located position in space relative to them, as long as it doesn’t shove them apart.. So, too, will any further sticky balls stuck onto the mass.
This permits the construction of arbitrarily large shapes with micron-level precision and some degree of geometric freedom, which is larger if you use multiple different sizes of balls. If the balls are millimeter-scale or smaller, you can get very substantial structures with reasonable strength. If surfaces are finished with non-sticky balls, those surfaces too will be precisely located.
“Voxel-based 3-D printing” is a name sometimes used for this sort of process.