In engineering education, the integration of disciplinary, societal, and ecological perspectives is facilitated by boundary crossing. This competency is cultivated through boundary-crossing learning mechanisms and is considered essential for students in interdisciplinary challenges in complex learning environments. However, limited research exists on students' perceptions of their own learning processes when developing the complex skill of boundary crossing. This paper, therefore, investigates boundary-crossing learning mechanisms, sub-mechanisms, and frictions that drive learning. The authors qualitatively analysed 720 reflection documents from 180 students in a 2nd-year interdisciplinary engineering master's course. The study reveals a progression from recognising strengths to calibrating expectations, identifying weaknesses, and positioning within meaningful activities. Identity (I) Concepts were used to examine frictions in this process, showing students engaged with themes such as self-knowledge, interdisciplinary teamwork, relationship building, and approaches to culture, structure, and planning. These insights may inform the design of interdisciplinary engineering education as transformational learning journeys.