First of all, great question! However, in order to answer it properly, we have to start by explaining the general terms first - that’s what Socrates would do, so if you’ve got a problem with that, take it up with him!
Soy sauce is a condiment that originated in China around 2500 years ago. Its current form, however, dates back 2200 years and is connected to the Western Han dynasty. Traditionally, soy sauce is made from fermented soybeans, a form of roasted grains, brine, and koji mold. The method of brewing Japanese soy sauce is a bit different, mainly because it lasts longer, and is called Honjozo. The production method, as well as the final product, varies drastically based on what type of soy sauce is being made, but the overall structure remains pretty much the same.
So, how Is Japanese Soy Sauce Made? Well, during honjozo, the combination of steamed soybeans, roasted wheat, and koji mold is fermented in barrels with some brine for a few months, allowing every flavor to develop and deepen, before bottling the final product. Once the fermentation process is over, the liquid is pressed and pasteurized before reaching your shelves (or the tiny sauce containers next to your sushi order).
The main difference between Chinese soy sauce and Japanese shoyu is the fermentation time. Shoyu is often left to ferment for 6-8 months, while Chinese soy sauce only takes a few weeks. The final result varies in color, the level of saltiness, and umami, not to mention in the naming system as well! While for Japanese shoyu light and dark are the indicators of the saltiness of the condiment, with Chinese soy sauce, they simply indicate the color of the sauce itself. Not only that, but light Chinese soy sauce can sometimes even be the saltier option than the dark one!
And is soy sauce bad for you? Well, that might be one of the most frequently asked questions regarding this stunning condiment. The truth is, soy sauce is pretty high in sodium (as most salty food is) so in high quantities, it can definitely cause harm (but then again, so can most food). The good news is, when something packs as much flavor as traditional Japanese shoyu, you don’t really need to have all that much of it - definitely not enough to cause harm. So no, a single dip of your California roll or a single dash of soy sauce into your soups and stir-fries won’t cause any damage (unless you suffer from a condition that requires a low-sodium diet, of course).