The worst is salt water. The aggressive solution in splash water is simply poison for any unprotected metal. There is plenty of it, especially on the bicycle drivetrain. Even if some chains and sprocket sets are coated, the corrosive moisture creeps into every crevice and does its evil work there. With good lubrication in the right order, you can beat the rust: Lubricate your cleaned chain link by link with viscous chain oil in winter. It is better to apply a thin layer of oil several times than too much once, allowing it to creep into the bearing collars and roller gaps with each pass. Then wipe off any excess oil with a cloth so that the outside of the chain is dry. Then apply a layer of spray or liquid wax to the outside of the chain. This keeps the lubricant in place and prevents it from washing out quickly. In addition, the wax dries on the outside and less dirt adheres again.
Proceed in the same way on the derailleur sprocket set: Clean, oil, allow to penetrate, wax. The joints of the derailleur, derailleur pulleys and pull levers can also be treated with spot oiling and a preservative layer of wax. However, always make sure that the brake track, pads and discs remain absolutely free of grease and wax. It is even advisable to degrease brake flanks and discs directly with spirit and lightly sand the brake pad surfaces, for example with sandpaper. This improves the grip of the brakes.
Unhook externally routed, interrupted shift and brake cables. Slide plastic liners over open cable sections. Drizzle oil or, even better, a few drops of Tiptop's Special Bike Fluid between the cable and housing, as it is specially designed to prevent ice. A blob of viscous grease at the end of the cable housing also seals against moisture ingress. Tiptop Fluid can also be used to free up frozen cables or derailleurs: the "magic agent" works like de-icer for car door locks. The penetrating oil content lubricates on the spot, while the alcohol it contains reliably dissolves gumming and ice.
A belt drive also requires care, it is low-maintenance but not maintenance-free.