This synthesis report examines interoperability challenges across European University alliances, focusing on creating virtual campuses that enable seamless student mobility and digital education. The report analyzes eight key use cases along the learner journey, from discovering opportunities to earning credentials. Through data from over 40 alliances, it identifies three main interoperability approaches: centralisation, broadcast, and decentralised strategies. Key findings highlight that 75% of alliances demonstrate emerging focus on interoperability, with priorities including course discovery, user identity management, and credential recognition. The report addresses technical, legal, semantic, and organisational challenges while proposing solutions aligned with European frameworks like eIDAS and GDPR.