## Virtual layer

For a dynamic solution (create a point at the lower left corner of the map canvas that changes when the canvas changes), create a Virtual layer with this query and replace `poly` with the name of a polygon layer:

```
SELECT MakePoint (MbrMinX ( Extent (p.geometry)), MbrMinY (Extent (p.geometry))) FROM poly as p;
```

## Limitation: get map canvas extent

The difficulty is to get the map canvas extent in a similar way as with the variable `@map_extent` in QGIS expressions. In [SpatialLite's SQL functions reference list][1], I don't see any function to get the map canvas extent (I posted a new question for this: https://gis.stackexchange.com/questions/439295/get-extent-of-the-current-map-canvas-in-a-virtual-layer-query-in-qgis).

That sayed, you first must have a polygon (canvas bounding box) for the query.

Unfortunatley, getting the coordinate values of the point you're looking for with `x(point_n(@map_extent, 1))` works fine e.g. in the label dialog, but does return only `NULL` in the attribute table/field calculator. Somehow this makes sense as the layer has it's own extent, but has no connection to the (current) map canvas extent at all (canvas can change, but layer remains unchanged). So you can't use a virtual field to create coordinate values you can access in the SQL query.

## Geometry by expression

What you can do - but then it's not a dynamic solution any more:

You can use the same expression as with geometry generator, but use Menu Processing > Toolbox > [Geometry by expression][2]. It creates the point as an actual geometry, not just symbology. 

You must do this based on any input vector layer (point, line or polygon). The point will be generator on a new layer.

See also here for [similarities and differences between Geometry generator and geometry by expression][3].


  [1]: https://www.gaia-gis.it/gaia-sins/spatialite-sql-latest.html
  [2]: https://docs.qgis.org/3.22/en/docs/user_manual/processing_algs/qgis/vectorgeometry.html#geometry-by-expression
  [3]: https://gis.stackexchange.com/a/392619/88814