It's muddy but the Executive isn't making laws it makes regulations constrained by and implementing the laws passed by Congress. It's all nominally rooted in some law the Congress passed and instead of just making those interpretations known when they sue you because you're using a financial instrument to defraud people there's a whole process of making it known how the Executive believe the old laws relate to new situations. Congress has neither the bandwidth nor the knowledge to keep abreast of every novel maneuver around the law so they say this type of thing is illegal and this agency is in charge of saying what type new things are.

A great example of that are with various toxins and pollutants, there's no system in which we can go through the whole process of making a new law every time we discover that some miracle chemical is giving people giga-cancer. Instead Congress tasks an agency full of experts to decide what safe levels of the giga-cancer causing chemical is and makes sure we only ingest slightly below the LD50 of that so we can statistically live.