Thanks for the question, @MQS! First off, you can absolutely use Cesium with free and open source data! Cesium for Unreal can load 3D data using the 3D Tiles specification, as well as raster overlays sourced from TMS, WMS, or WMTS APIs. OpenStreetMap’s wiki has a list of raster providers, most if not all of which should work with Cesium for Unreal: Raster tile providers - OpenStreetMap Wiki.
3D data is also easy to find, but needs to be in a format Cesium understands. For terrain, heightmaps are widely available (for example, from USGS) and are especially suited for the quantized-mesh format that Cesium for Unreal supports. Someone has assembled a nice GitHub repo listing tools for working with quantized-mesh, if this is the route you choose to go: GitHub - bertt/awesome-quantized-mesh-tiles: Awesome implementations of the quantized mesh tiles specification.
For buildings, you can use the OSM Buildings dataset, as long as you get the data into 3D Tiles. 3D Tiles is the much more recent and more versatile specification for getting 3D data into Cesium, and there’s a similar list of tools for working with it here: GitHub - pka/awesome-3d-tiles: Awesome 3D Tiles. Particularly, someone has done some work on a tool for converting the OSM dataset to 3D Tiles, though I haven’t used it so I can’t personally vouch for it: GitHub - kiselev-dv/osm-cesium-3d-tiles.
Whichever approach you use, once you have your tileset, all you need to do is throw it up on a web server somewhere, or put it in a local directory, and point Cesium for Unreal at it.
Of course, if you’re looking for an alternative to hosting and converting that data yourself, I’d recommend Cesium ion! Not only can you use the Cesium World Terrain and Cesium OSM Buildings datasets that we’ve put together and polished up yourself, but you can also upload data you find around the internet and convert it into 3D Tiles. You can see here the formats ion supports: Tiler Data Types and Formats – Cesium. If you find data in a format that we don’t currently support, please reach out and we can work with you on getting it in!
We also offer Cesium ion Self-Hosted if you’d still like to host the data yourself. We optionally offer Cesium World Terrain and Cesium OSM Buildings as part of Self-Hosted. Definitely contact us if this is something you might be interested in!
Finally, on your point about the Apple maps API, that’s a great suggestion! I’ve passed it along to our data team to investigate.
Thank you for all your questions and suggestions!