Visualizing region unions

Amplifying on the answer by e.doroskevic

The Head of RegionUnion is RegionUnion which is not a Graphics primitive.

Head@RegionUnion[Disk[{0, 0}, 2], Disk[{3, 0}, 2]]

(*  RegionUnion  *)

RegionQ@RegionUnion[Disk[{0, 0}, 2], Disk[{3, 0}, 2]]

(*  True  *)

Use RegionPlot

RegionPlot[
 RegionUnion[Disk[{0, 0}, 2], Disk[{3, 0}, 2]], AspectRatio -> 4/7]

enter image description here

Another possibility is BoundaryDiscretizeRegion:

BoundaryDiscretizeRegion[
    RegionUnion[{Disk[{0,0},2],Disk[{3,0},2]}]
]

enter image description here

If you want a better discretization, you can use PrecisionGoal/AccuracyGoal:

BoundaryDiscretizeRegion[
    RegionUnion[{Disk[{0,0},2],Disk[{3,0},2]}],
    PrecisionGoal->6
]

enter image description here

Input

RegionPlot @ RegionUnion[{Disk[{0, 0}, 2], Disk[{3, 0}, 2]}]

region plot output