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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.

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Basically: GIS tools (including QGIS) usually default to EPSG:4326, as do global databases such as OSM.
Google Maps and OSM map services (e.g. terrestris) use EPSG:3857, however.

And always be aware that sadly some use coordinate directions (LatLng or LngLat) very liberally, sometimes independent from what the EPSG would require.

See EPSG 3857 or 4326 for GoogleMaps, OpenStreetMap and Leaflet for more details.

If your data has no projection info attached, QGIS will warn you about it. Take this seriously. Yes, it will try and interpret the data as EPSG:4326, but if it actually isn't EPSG:4326, any work you do on the data will be wrong.

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To be sure of which projection your QGIS uses check the project properties via: Project > Project properties > CRS, and set the projection to your requirements.

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