There is no one configuration that describes a conventional septic system perfectly. The conventional septic system has evolved over time as a way to process waste water whether it is from a commercial establishment or a residential location. Depending on the age of the septic system it will most likely contain characteristics based on the standard practices that were prevalent at the time that it was installed. Conventional septic systems that were installed before there was local regulation in place could be made up of almost anything. Once the need for regulation was identified there was most likely already the practice of using single or double 500 gallon concrete tanks. The multiple 500 gallon concrete tank configuration with a gravity fed subsurface leaching field is widely used still to this day.
Wikipedia has this explanation for “septic tank” https://en.wikipedia.org/wiki/Septic_tank