The Unmanned Aircraft System Traffic Management (UTM) system is designed to autonomously coordinate dense Unmanned Aerial Vehicles (UAVs) within shared airspace, ensuring both the efficiency and safety of aerial traffic. With the rapid proliferation of UAV applications, autonomous UTM systems have become increasingly essential, motivating various stakeholders to develop their distinct UTM solutions. However, due to the lack of common guidelines, these emerging solutions exhibit substantial incompatibilities, which hinder the transferability of existing techniques and the overall standardization of UTM. To address the fragmentation, this paper provides a systematic survey of existing UTM research and identifies commonalities across various UTM systems. Specifically, this paper summarizes core UTM service modules and groups them with similar objectives, thereby proposing a unified UTM framework with four layers: Fundamental Infrastructure, Pre-flight UTM, In-flight UTM, and UTM Application. Based on the framework, existing solutions for each module are reviewed in detail. Furthermore, this paper draws analogies between UTM systems and more mature transportation systems, like railways, to identify transferable solutions and derive UTM future trends. This survey aims to clarify the current state of UTM research and provide guidance for future studies in this field.