A wharf is just a dock. Where ships are loaded/unloaded with general cargo.
If the wharf is rail-served, it'll be handling cargoes that can be transferred from rail to ship or vice versa. This may mean different things at different points in history.
Earlier in the 20th century, a lot more "break bulk" and general cargo would be loaded on ships, so boxcars could be transloaded at a harbour warehouse. Modern era, that sort of stuff would be containerized and handled as intermodal. Large oversize industrial loads (turbines, generators, transformers, etc.) would be a good choice for loading at a dock, and bulk commodities are probably the most commonly rail/ship transferred cargoes, but require special handling facilities (grain elevators, coal and ore docks, etc.)
There's no special "ops" magic to it, it's an industry where cars are loaded or unloaded.
Not a wharf, but a car float. Some railroads used car floats (barges moved by tugs) and car ferries (larger self-propelled vessels.) For many years. The harlem river RR had some small yards in NY that had no connection to the rest of the rail network, except by water. Car ferries, and floats required special loading facilities with a track, or tracks, hinged at the land end and supported by a float at the other. The photos show my N-scale tug, and car float.