There are different models of sharpeners for sharpening the blades of a safety razor (1). In the past, people used a razor strop on which they pushed the knife back and forth.
You can also use a concave oil stone to sharpen the blades. [MOT]
(1) DURBIN G., Wig, hairdressing and shaving bygones, Oxford, 1984: 21.
Safety razor sharpener MOT V 2018.0253 a-c3