## 2 Research Methodology  
Because of UAVs' widespread applications in various industries, as indicated in the Introduction, extensive literature in this field has arisen. This fact in turn creates a challenge while conducting a thorough review of UAVs' internal structures and composers. To address this challenge, and to better guide interested audiences through the different aspects of UAVs, the systematic approach adopted here consisted of dividing a UAV into five main sets as follows:  
1. Control: This set includes navigation systems, flight control, autopilot, collision avoidance, target tracking, fail-safe, motor speed, and other related systems that are dedicated to managing and directing a UAV's flight.
2. Computing: This set includes the computational elements including data processors, onboard computers, data loggers, and all computing platforms responsible for the execution of different algorithms.
3. Communication: This set includes the information exchange between a UAV and external parties (i.e., for remote control options) performed through different communication modules (e.g., Bluetooth, Wi-Fi, Long Range (LoRa) modules, etc.).
4. Sensory: This set includes data captured from internal (e.g., UAV's power consumption and temperature), as well as external (e.g., altitude, pressure, and wind speed) environments held through UAV-embedded sensors.
5. Power: This set includes the energy sources (e.g., battery/solar cell drives), power distribution, and power management systems with the related circuitry to provide UAVs with optimal power for proper overall functionality.  
Technically speaking, this research began by searching for relevant keywords, such as flight control in UAVs, avionics integration, UAV autonomous navigation, different power source types for launching UAVs, communication modules in UAVs, and others. In the first step, the corresponding results formed a large database, which was decomposed later according to the previously mentioned five main sets. Concerning the aim to inform readers about different aspects of UAVs, the conducted structure was developed in the form of relative answers to the following research questions (RQs):  
RQ1: What are the different subsystems within a UAV?  
RQ2: Is there any hierarchy between the different subsystems?  
RQ3: Is there any integration between the subsystems?  
RQ4: How can UAV reliability be enhanced by means of multiple sensory systems?  
RQ5: What are the programming languages for different computing systems?  
RQ6: What is the relationship between sensory and computing systems?  
RQ7: How is the interdependence between subsystems managed?  
RQ8: What are the standard UAV communication protocols?  
RQ9: How can fail-safe be ensured in emergencies?  
RQ10: How can limits be set for motor speed and UAV maximal altitude?  
The answers to the raised questions consequently shaped the general overview of this manuscript, allowing thorough knowledge to be gained about the various facets of UAVs. The methodologies inside each set are mutually compared and assessed. Ultimately, concerning the resultant tabulated performance and other metric data for each set, the relevant discussed project highlights the contribution of this paper's outcomes to the body of knowledge of UAVs.