Virtual Layer in QGIS
As explained in the QGIS official documentation, a virtual layer is "A special kind of vector layer that allows you to define a layer as the result of an advanced query, using the SQL language on any number of other vector layers that QGIS is able to open"
A virtual layer does not carry data by itself and is basically a "view" of the data.
This tutorial refers to the previous tutorial at • Rule Based layer Symbology & Labeling in ...
Explore writing queries for virtual layers