Select the holes in a vector shapefile in QGIS

If you dont mind a two step process:

1. Run "Delete Holes" from processing toolbox on your "PolygonWithHoles"
2. Run "Difference" and use your "Cleaned" (Result from Delete Holes) as Input and your "PolygonWithHoles" as Overlay:

Result:

If you wish a one-step solution, you could also use "Geometry by Expression" with this expression:

difference(make_polygon(exterior_ring($geometry)),$geometry)

or a "virtual layer" with this query:

select difference(make_polygon(exterior_ring(geometry)),geometry) from PolygonWithHoles

Both will return a multipolygon of the holes.