This study explored how agency is expressed when undergraduate students cross curricular boundaries to learn software development by realising affordances enabled by resources on online platforms. The study employed a qualitative research design based on individual stimulated recall interviews with 27 computer and software engineering students. The hybrid thematic analysis employed an ecological framework in which the actions the students performed during software development tasks were interpreted in relational terms. The results reveal multiple interrelated affordances that were realised as the students learnt by using various resources on different online platforms. Agency was expressed primarily through students pursuing specific objectives related to their learning across platforms and curricular boundaries and was shared and distributed across people and the environment. The findings highlight the idea that boundaries are formed and crossed based on how people experience the environment, which, in turn, is enabled or constrained by the resources used.