Are special cables always custom-made?

No. Many special cables are standard catalogue products with fixed construction, conductor size, core number, insulation, sheath, and voltage rating.
They are “special” because they are used in demanding applications such as cranes, elevators, industrial automation, fire alarm systems, marine panels, PV systems, or machine tools.
True custom cables are only required when the buyer needs a non-standard combination of core arrangement, shielding, armour, compound, colour, voltage rating, or communication element.