This question already has an answer here:
Google/Bing Maps etc. or web mapping applications use WGS84 coordinates. When you moves the mouse on map, you see WGS84 latitude and longitude. Although coordinate system is WGS84, map is projected in Web Mercator on the screen. Spatial data with projected coordinates is in 2D and it's easy to show on screen, because it's in 2D and already a plane coordinates.
I wonder what is projection system QGIS uses for spatial data that has just geographic coordinates.