I’m have drawn a circle in my 3d world, now I want to calculate a random point inside this circle. But not in the while circle but in a certain area inside this circle.
The random point should be inside the red area (radius divided by 2)
Uhm just use Random.insideUnitCircle and multiply by your desired radius and you get a random vector which length is between 0 and your desired radius. Of course the origin is at the center of the circle. If you want the center somewhere else, just add that vector to your desired center
Vector2 randomPoint = centerPoint + Random.insideUnitCircle * radius * 0.5f;
Note that * 0.5f does cover your requirement of “half the radius”.
ps: You said in your “3d world” however a circle is just a 2d shape. You can use 2d shapes in a 3d world however there are endless ways how such a circle could be oriented in 3d space. If you’re actually not looking for a circle but a sphere, just use Random.insideUnitSphere instead.