I want to fill the missing space and create a polygon out of it. How do you do that with QGIS?
The missing space is the white part between the selected/yellow polygons.
Here is a snapshot:
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In QGIS you can achieve this just activating the Avoid Overlap option inside the Snapping Options.
Activate the Snapping and open the Snapping Options:
Here, select the option Advanced Configuration and click, to activate, the options Topological Editing.
Activate (check it) the layer(s) that delimitated the empty space and, finally, check the option Avoid Overlap in the layer(s).
Now you can just draw a new polygon with its nodes inside the surrounding polygons and the system it will avoid automatically any overlap creating a new geometry that covers only the empty space.
Since the Version 3.14 QGIS has a new Snapping toolbar and the Overlapping option has a dedicate icon: