Cloud services are the actual infrastructure. Open source is the blueprints and building materials of our digital infrastructure. Infrastructure should be built from open source, but it is important to understand the difference between software and a service, because there is a person in the middle, the one operating the service. There will be no digital sovereignty just with open source, only with transparent services. The combination creates the necessary infrastructure and resulting sovereignty.
A simple but important example is email. If you ever worry about a government getting overbearing, you’ll need a way to coordinate efforts of protest. Email and other communication services are clearly utilities that need to be transparent and neutral. Given the complexity of these services, it is unlikely that a grassroots efforts of citizens can provide them. Thus, either the government operates the services (not necessarily a good idea) or they are regulated like the energy utilities.