A uniformly charged object, shown as a red circle, is placed inside a conducting metal box along with two conducting plates. The walls of the box and one of the plates are connected to the Earth, that is, to ground. The other conducting plate is not connected to any object.
Move the dark red charge between the plates to determine which plate is grounded and which plate is disconnected.
Notice how the the pale red and pale blue circles change as you move the charged object. These circles represent positive and negative charge, respectively. These circles do not represent charged objects nor are they represent the literal location of charge. Charge is, in fact, distributed smoothly on the surface of a conductor and does not form circular lumps. The circles on the screen are merely a visual representation of the charge density in the vicinity of the circle.
Where does the charge on each of the conductors come from as you move the dark red charge?
About the geometry. Since we again have a two dimensional simulation, you have to imagine that on-screen objects extend out of and into the computer screen without changing shape. Circles are therefore cylinders and rectangles are long plates. Two dimensional and three dimensional geometries, that is cylinders and circles, can give similar qualitative behaviors but different quantitative results.