There are billions of IoT devices worldwide, and this number is set to skyrocket in the coming years. Among this vast landscape, ultra-constrained devices are becoming a dominant force, shaping the future of IoT. Given their sheer volume and highly specific applications, these devices must be maintenance-free, self-sustained, affordable, autonomous, and seamlessly integrated into their environments. These characteristics define what are often referred to as energy-neutral, zero-energy or ambient IoT devices (as coined by 3GPP). For simplicity, the term energy-neutral device (END) will be used to emphasise their pivotal role in fostering sustainable IoT ecosystems and driving societal progress.