This study explores the origins of network-based and corporate-based unionism in the gig economy. To explain variations in platform unionism, I propose a framework to understand how organized labor activates embryonic solidarity through collective visions and solidarity practices. Utilizing this framework, the author examines the successes of the Rider Union (RU) and the Korean Federation of Service Workers Union (KFSW) in the South Korean food delivery sector, resulting in differential institutional outcomes, such as changes in industrial accident compensation insurance and collective bargaining agreements, respectively. Based on an analysis of 56 interviews, 4,043 online community postings, and 210 news articles, the study shows that the RU builds network-based unionism by tapping into fluid solidarity through inclusive visions and boundary-spanning practices, whereas the KFSU develops corporate-based unionism by activating solid solidarity through platform-specific visions and boundary-limiting practices. Furthermore, after establishing their bases within public spaces and platform boundaries, respectively, both unions extend their influence into different areas by adopting strategies from each other.