I'm trying to understand how the pictured device works. It's a Sonoff WiFi controlled electrical relay. I was told that it is designed to control mains voltage only, and cannot be used to control a 5v dc current. This doesn't make any sense to me, because I'm under the impression that the relay which ultimately controls the current works by simply completing and breaking a circuit using an electromagnet to move the switch. If so, this would mean that the current may practically be as low as I want, as long as it's below the maximum voltage and amperage rating for the relay.
Am I correct in my belief that a device like this can be used to control a 5v dc current without modification, or do I need something specifically designed for lower voltages? If I need something different, then what do I need, and why?