This study investigates how ninth-grade students in Swedish compulsory school describe and explain a technological system: the wastewater system. The analysis focuses on students’ verbal explanations while illustrating their self-drawn models of the system. Eleven students (aged 15–16) participated through semi-structured individual interviews. Transcripts and models were analysed using Hallström et al.’s (2022) classification model for system understanding and thematic analysis. The results indicate that most students were able to identify the system’s purpose, namely, the collection and treatment of domestic wastewater, and describe components such as household outlets, sewer pipes, and treatment plants. However, their descriptions were largely linear and focused on visible components, such as inlets and manholes. Few references were made to energy flows, information control, system boundary, or interdependencies with other systems. Most students’ reasoning remained at the Multistructural level; only two demonstrated relational understanding, and none reached an extended abstract level. The thematic analysis revealed that students faced difficulties in understanding temporal processes, feedback mechanisms, and the consequences of system failures, highlighting difficulties in grasping system complexity. The study calls for instruction that explicitly makes hidden structures, interconnections, and sustainability aspects visible in technological systems. It proposes combining student-generated drawings with visualizations, simulations, and structured reflection to promote deeper and more transferable systems thinking in technology education. Although grounded in a Swedish context, the findings and suggested teaching strategies may inform broader educational settings and contribute to strengthening systems thinking as a core competence in technology education globally.