Comment by jvanveen

It is basically an openstreetmap viewer plus a simulator layer that turns the data (https://versatiles.org/) into 3d at a certain level of detail. The Netherlands have very detailed geo data-sets (https://www.pdok.nl/), which are used to augment the 3d representation with stuff like point-cloud rendered buildings (3DBAG), crop-rendering, benches, etc.. Basically, it tries to combine as much public data as possible, to represent a city. The simulator is then used to be able to navigate through it with an avatar and to build basic stuff (Secondlife prim-style + Lua scripting engine).