I would give you 1 word, but this word turns into another word later.
I will not use your basketball. I need to simplify this further.
A. No shadow does not give you any 3-dimensional clue.
B. But adding shadows gives you visual parameters on things that are in front.
C. Or behind.
But it also gives you references to distances. D, E, F.
There are different types of shadows.
G. Cast shadows, parallel shadows.
H. Perspective shadows.
In the end, these two types of shadows give you a clue on what is the relation between the object and the background. The first tells you the background is parallel to the circle, and the second one tells you that the circle is perpendicular to the circle.
I. But there are also shadows self-projected in the object, which gives you the internal shapes of the object.
And here is when I want to switch the word shadow for a specific tool you can use.
Different types of gradients will give you a thing on what is the 3D shape of the 2D figure
J. It could be a sphere
K. A cone
L. The top view of a helix
M, N. If you start combining gradients you now start having different materials.
O. You can make more complex things. But in the end, the thing that gives you shape is the shadow (gradients).
Explore more about materials here.
Sure, there are other elements like perspective and size.
But the look that a gradient, when used as a shadow, is the main factor.
P. A flat shadow does not look 3D at all
Q. A cube with some perspective
R. With some gradients
A Bonus, Blending modes.
A quick way to apply a shadow on an object, but maintain the editability of both, the shadow and the main shape, is the multiply blending mode.
S. No blending mode
U. One over the other
You could explore more ways to combine them.
U. No blending mode
V. Add blending mode
W. Here is another thing. One shape inside the other. In order to really control the shape of my sphere, I in reality put the gradient inside a circle.
One important thing about shadows is directionality. For example in photography, the angle in where you put the light makes a portrait flat or dimensional.
So, keep exploring.
In Inkscape, you have a couple of tools that can help you to make gradients.
Although my interface is in Spanish, the location and icons are the same in English.
1. One is the gradient fill tool and the other is the grid fill. (The names might be different, feel free to correct me) Just select an object and apply a fill.
2. Open the object panel so you can control the exact color of the fill.
3. A fill is handled by nodes. Use the node editor to edit a specific node (4) and change its values on the object panel (5).
The Mesh fill works similar to the gradient fill, but you have different nodes on the outer rim of the object.
Look for tutorials on how to use nodes and fills. For example https://www.google.com/search?q=inkscape+gradients
And here is a blur tool
P.S. As you first asked about "the theory" I only illustrated the point with "X" program, it does not matter which one. The only gradient I think you do not have on Inkscape is (L) The last images are done in Inkscape.
On Inkscape, I do not use any filter on the filters tab. I am only using in these images gradients blur and blending modes. It is not about having a ton of tools, but only a few and controlling them.
Some other tools to explore
Make, for example, a text.
Convert it to path (Path>Object to path)
Make a shape with the desired perspective.
CLick the "no longer text", click the domain
Extensions >Modify Path> Perspective.
Select two simple paths
Extensions > Generate from Path > Extrude