If you want to highlight an area on your map in QGIS you can easily do it by duplicating a layer and then using an inverted polygon + shapeburst fill. Take a look at the video to see what I mean.
You don't have to do it using two layers but I do that tin this video because sometimes I still like to have the area boundary showing, but it's up to you.
See my intro and intermediate QGIS courses for more: [ Ссылка ]
