There is a new mode for 3D scenes - globe view - in addition to the traditional 3D view based on a projected CRS.
Any map layer type can be used as a 2D texture for the globe, and 3D renderers of tiled scene layers and point cloud layers are supported.
Globe view uses the ellipsoid of the project to generate globe mesh. It is even possible to use other celestial bodies, such as Mars or Moon, if the project's CRS is set up accordingly.
More info:
- QEP #301 - Large scenes and globe in QGIS 3D
- Cesium blog post