Engine heaters come in two flavors - oil pan heaters or water jacket heaters. The most popular are the water heaters, which I'm pretty sure is what is in your Journey. These units pop into the side of the engine block and heat up the coolant surrounding the cylinders. In the process it heats up the steel and some of the oil in the engine.
None of these units have thermostats but they aren't of a sufficient wattage to ever overheat anything. You won't boil your coolant or your oil so there's no worries. If you ever plug in an engine block heater when the coolant is drained the electric heating coil will overheat (because there's no coolant around it to remove that heat) and it will fail. It may fail in the open circuit position or it may short out to ground, which will trip your breaker.
Mark & Leann Quasius
2016 Cornerstone 45A
2020 Jeep Gladiator Rubicon
2021 Jeep Wrangler Rubicon