Some vector sources have multiple layers while many have only one. Shapefiles
for example have only one, and the single layer gets the file name with no path
and no extension. GDAL provides a quirk for shapefiles in that a directory may
act as a data source, and any shapefile in that directory acts like a layer of that
data source. This is a little like the one-or-many sleight that exists for raster
data sources with subdatasets (there's no way to virtualize single rasters into
a data source with multiple subdatasets, oh except by using VRT....)
See vapour_sds_names for more on the multiple topic.