Sub-THz communications are envisioned to unlock ultra-high data rates, ultra-low latencies, and massive connectivity in future wireless systems by utilizing bandwidths of up to tens of GHz. This paper presents the holistic perspective on sub-THz communications developed within the European sixthgeneration (6G) flagship project Hexa-X-II. According to this perspective, successfully deploying sub-THz systems will require addressing several physical-layer (PHY) challenges associated with the harsh signal propagation and hardware limitations. In addition, sub-THz systems will drive advancements in essential wireless applications beyond communications, such as joint communications and sensing (JCAS). In this context, we present a collection of technical contributions and key findings from the Hexa-X-II project that have shaped this perspective. In contrast to existing works, these efforts collectively tackle the fundamental PHY challenges of sub-THz communications, i.e., understanding and modeling of radio propagation, radio-frequency (RF) power consumption and complexity, and hardware impairments. To achieve this, we present several results across three core areas, i.e., signal propagation and channel modeling, RF transceiver design, and PHY enablers, offering useful insights into the development of sub-THz systems. Lastly, we provide an overview of sub-THz JCAS, emphasizing it as one of the most promising applications in the sub-THz range.