## 1 Introduction
In [55], the research considered an overview of anti-collision technologies for UAVs with the associated types of sensors, whereas in [56], the focus was on UAV threat models, with security and privacy aspects. Divergently, in [57], the focus was on search and rescue operations driven by UAVs, whereas in [58], the review examined the deployment of UAVs for monitoring and inspection in the construction industry. Concerning the You Only Look Once (YOLO) algorithm used for real-time detection and classification of multiple targets, the work in [59] investigated the integration of YOLO with UAV technology and the corresponding practical applications (e.g., engineering, transportation, automation, etc.). Given that path planning holds significance in the context of drone autonomy, the authors of [60] reviewed the environmental representation as well as the path generation techniques for drones. Considering the potential of UAVs in warehouse management, a systematic literature review was conducted in [61], enriching the background of knowledge about the obstacles versus the adoption of UAVs in warehouse handling, unlike in [62], where the review primarily focused on the role of drones in flood management. Different UAV platforms for autonomous applications are reviewed in [63], which also presented the state-of-art and estimation techniques for UAVs, in addition to their correspondent flight phases. As can be noted from the literature survey encompassing the modern reviews in [46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63], the majority of these articles actually focus on the UAV domain of usage and application fields. In other terms, there exist no reviews about the drones themselves, involving different architectures, structures, avionics, and software-to-hardware branches (i.e., the results of algorithms over the behavioral mechanism of sensors). Forwardly, as a matter of proof, and despite the extended application fields for UAVs such as concentrating solar thermal system planning and operation [64], intrusion detection [65], forest ecology [66], forensic civil engineering [67], plant-scale growth estimation [68], and photovoltaic plant diagnosis and maintenance [69; 70], the available related reviews provide a less detailed informative background about the realm of UAVs. To address this issue, this paper aims to critically review and assess the different UAV facets, decomposed into five major sets including flight control and navigation, communication and remote control, computing and data management, sensing and monitoring, and power and energy management. The intended structure of this review aims to establish an informative background concerning the UAV compositional realm, instead of possible field applications, for UAV design processes. The rest of this paper is structured as follows: Section 2 presents the research methodology used to obtain data, Section 3 outlines a general UAV operational overview in which a comprehensive synopsis of UAV operations is presented, Section 4 elucidates the actual review of different sets (i.e., subsystems) within a UAV, Section 5 discusses the obtained results, Section 6 highlights an actual UAV-based future project design with a corresponding link to this review, and finally, in Section 7, conclusions are derived.