Sun energy is main external input. But not exactly correct. Consider meteorites, for example. A lake is good example. Sunlight input, water flow in and out.
Open systems are common in nature.
No energy or matter can cross boundaries. No interaction with anything outside of that system. Would be hard to find such a system on earth. Dynamic Equilibrium refers to a thermostat - like concept, as mentioned in the river dynamics example previously. Systems where inflow tends to balance outflow are tending toward what is called a steady-state under dynamic equilibrium.