The brine is actually magnesium chloride, a byproduct of extracting salt from sea water.
Chinese Nigari Tofu is likely to be extra-firm, so in the west the impression is that all Nigari Tofu is extra-firm. The term, though, just refers to the curdling agent that was used. The resulting tofu can be any of the textures that tofu comes in (silken, cotton, extra-firm etc.)
Most other tofu made now uses Calcium sulphate, because it is less demanding.
Nigari curdles soymilk very quickly, As well, it will react differently based on the temperature of the soymilk, and how old the soya beans being used are. Consequently, you have to watch it closely, which is not desirable in mechanized production.