No battery change and no separate power supply are required to obtain measured values such as the current temperature. This means that sensors can be better positioned and provide continuous data.https://www.notebookcheck.net/Sensor-without-power-or-battery-Energy-comes-from-the-environment.796083.0.html
it is not "circuit that can harvest energy from magnetic fields" in the photo, it is energy management PCB for external energy harvester
and energy isn't "coming from the environment", harvester is fixed around an actual live wire, powering the stuff you need a sensor for