In Arcgis, there is a way to offset graphically the contour line of a polygon like below. I'm trying to do the same thing in a styling method (i.e the solution can't be by buffering physically the layer). QGIS does have an offset option, but it's an XY type offset that moves the position of the line.
Way 1: use the geometry generator fill style. Just get in the box buffer($geometry,[your buffer distance]). The only difference is that Arcgis offsets the line whereas this solution creates a buffer, thus rounding the edges...
Way 2 : exact replica of Arcgis, inspired below by @iant answer : create in the style another style "Outline : Simple line". In this style, there is the offset box i was looking for ! It does exactly the same as in arcgis.