Creating polygon from hole in QGIS?

I recommend Digitizing Tools plugin.

Once installed, you will find a new toolbar. The eighth from the left of this toolbar, there is an icon called Fill ring with a new feature (interactive).

For you to fill all rings at once;

1. Switch to Fill all rings in selected polygons with new feature mode by clicking on a small triangle (right-hand-side).
2. Start editing mode and select polygons you want to work on (you can select all polygons).
3. Click on the tool icon.
4. A small dialog box will appear and asks you to give a new attribute value. This value will be given all fillings, so that it helps you to identify these new features in the attribute table. Hit [OK] to close.

In QGIS I can suggest using a "Virtual Layer" through Layer > Add Layer > Add/Edit Virtual Layer....

Let's assume there is a layer "layer_with_holes" with its corresponding attribute table, see image below.

There are possible two cases.

---

Case I. When holes that are located on the polygon edges should not be included.

With the following query, it is possible to create a new polygon layer from the empty gaps inside the polygons.

-- generate series
WITH RECURSIVE generate_series(cat) AS (
SELECT conf.start
FROM conf
UNION ALL
SELECT cat + conf.step
FROM generate_series, conf
WHERE cat + conf.step <= conf.stop
),

-- config
conf AS (
SELECT
1 AS start,
1 AS step,
SUM(st_numgeometries(st_difference(make_polygon(st_exteriorring(l.geometry)), l.geometry))) AS stop
FROM "layer_with_holes" AS l
)

-- query
SELECT cat, st_geometryn(st_union(st_difference(make_polygon(st_exteriorring(l.geometry)), l.geometry)), cat) AS geom
FROM generate_series, "layer_with_holes" AS l
GROUP BY cat

The output Virtual Layer with its Attribute table will look as follows.

---

Case II. When holes that are located on the polygon edges should be included.

With the following query, it is possible to create a new polygon layer from the empty gaps within the polygons.

-- generate series
WITH RECURSIVE generate_series(cat) AS (
SELECT conf.start
FROM conf
UNION ALL
SELECT cat + conf.step
FROM generate_series, conf
WHERE cat + conf.step <= conf.stop
),

-- config
conf AS (
SELECT
1 AS start,
1 AS step,
SUM(st_numgeometries(st_difference(make_polygon(st_exteriorring(l.geometry)), l.geometry))) + st_numgeometries(st_difference(make_polygon(st_exteriorring(st_union(make_polygon(st_exteriorring(l.geometry))))), st_union(make_polygon(st_exteriorring(l.geometry))))) AS stop
FROM "layer_with_holes" AS l
)

-- query
SELECT cat, st_geometryn((st_union(st_union(st_difference(make_polygon(st_exteriorring(l.geometry)), l.geometry)), st_difference(make_polygon(st_exteriorring(st_union(make_polygon(st_exteriorring(l.geometry))))), st_union(make_polygon(st_exteriorring(l.geometry))))), cat) AS geom
FROM generate_series, "layer_with_holes" AS l
GROUP BY cat

The output Virtual Layer with its Attribute table will look as follows.