Press Shift/F2 on a books Tag cell, this will pop the Tag Editor where you can add tags and apply previously created ones, viz:
Or use the Metadata Edit Dialogue, there's a button there to do the same thing.
Virtual Libraries are based on metadata search expressions.
See Calibre User Manual -
The search interface and
Virtual libraries — Tutorial
If you click the sprocket icon at the left end of the search bar it will open up the "Advanced Search" dialogue, which is a bit of a misnomer, it is more like a Search Wizard.
Having formulated and done a search you can create a VL using that search expression just used
BR