The way they go about this is still a bit weird / government like. Individual municipalities write tenders for software companies to contribute to these projects. The Vereniging Nederlandse Gemeente is hosting the specific committees that are in charge of product vision / architecture. But I am not sure if there is a way for individuals to contribute to these projects right now. So the source is Open Source, the process very much not yet.
If you are Dutch and looking to work on awesome gov-tech send me a DM.
I see some communities, seems like each community has their own setup, some of them have github links.