For questions about the geometry generator symbol layer style in QGIS.

In QGIS, a geometry generator symbol layer allows to use expression syntax to generate a geometry on the fly during the rendering process. The created output works for visualization purpose only and is a special form of symbology, not an actual geometry to be used in further geoprocessing. However, expressions used to create geometry generator symbology can be used to create actual geometries using Menu Processing / Toolbox / Geometry by expression.