Formal and informal STEM learning settings are often assumed to interconnect, as conveyed by the widely used metaphor of the STEM learning 'ecosystem'. While this metaphor usefully conveys the sense of a complex system that comprised interconnected, differentiated parts, it has been critiqued for struggling to engage with power relations and remains more of an aspiration/ ideal, rather than depicting the reality of current STEM learning systems. This paper critically reflects on how we might think about and conceptualise the relationships between the myriad of offers that comprise formal and informal STEM learning provision, with the aim of supporting more equitable and inclusive policy and practice. We employ a Bourdieusian conceptual lens, arguing that relationships between formal and informal learning organisations might be more productively theorised as a 'market'. To explore the purchase of these metaphors, we draw on qualitative data from an international research--practice partnership project conducted between university researchers, practitioners and youth from six makerspaces in five countries. Specifically, we focus on data collected via 230 interviews with 100 + young people, 28 practitioners and a small number of parents, plus 23 youth group discussions/workshops and 62 observations of practice. Analysis identifies four main types of relationship between schools and makerspaces (termed 'recruitment', 'outreach', 'hosted visits' and 'full-circle' relationships). Practitioners simultaneously valued maintaining a distance and separation between formal and informal settings (reflecting the distinct value afforded by makerspaces) and called for new and different relationships with schools. Analysis explores the affordances and limitations of both the ecosystem and market metaphors for understanding these relationships between formal and informal STEM learning settings. We argue that the market metaphor augments the ecosystem metaphor by offering a more critical understanding and engagement with equity issues and power relations pertaining to STEM learning systems and hence has greater potential to support and inform equity-orientated policy and practice across both formal and informal sectors.