What does this really mean?
Before enlightenment, the world is seen through the illusion of duality. It is believed that you (a human being) exists and is interacting with a multitude of objects that exist separate from yourself. This belief system has been historically given the label materialism. In this belief system, what is real is the atoms and physical matter that exists in the material world, and your mind (5 senses and thought) are side effects, created by the complex system that exists as your physical body.
Based on your questions, it seems as if you still believe this is the case. Before I answer your questions, it is easier to understand what I am saying if you reflect on your own experience first.
It is important to first pause to consider if you have ever experienced anything "physical" in your entire life. Seeing, hearing, tasting, touching, smelling, and thinking are all immaterial sensations that can not be given to another nor placed in a box. Have you ever experienced anything other than these sensations? If no, what proof can you find that something physical does exist? Is it possible to find proof?
Now, on to your questions.
Let's say I have a piece of apple in my hand. I see that it's an apple. I feel that it's an apple when I touch it. I can smell it as an apple. It also tastes like an apple when I bite into it.
Look over these sentences again. You are describing sight, touch, smell, and taste. You are not describing an apple. What you call an apple, is simply a label you place on a subsection of your sight, touch, smell, and taste. You are saying "this sight I identify as an apple, this touch I identify as an apple, etc."
What is happening is you are breaking down and dividing these sensations into multiple different concepts. When you are observing sight, you are only ever seeing one thing, sight itself. But then, we take sight as a whole and break it down into multiple subsections and concepts. What once was sight is now your hand, the apple, and the room around you. This process of breaking apart a single thing and classifying it as multiple is one way the illusion around you persists. You see your hand and the apple as two different things, but that is not true. At first you may think both your hand and the apple exist within sight, but both are simply the same sight you have broken apart into different ideas. Sight is what is real and exists, and you have imagined your sight to be objects, instead of simply sight.
How does the apple emerge only from interaction?
How does the apple exist only as interaction?
Interaction is only possible to occur if duality exists. You believe you are one thing. You believe the apple is another, separate from you. Believing both of these things is what makes interaction seemingly possible.
If there is no you, and there is no apple, can interaction occur? If there is simply sight appearing, is any interaction taking place?
By holding the false belief that you are "this" and not "that", you instantaneously create the belief in both your and the apple's existence. If you did not believe you were separate from the apple, would the apple still exist? Or would it simply be a part of you?
How is the apple like a sleight of hand that only appears solid, although it's virtual?
Now that Virtual reality technology is accessible to the masses, lets have a VR example.
Image you put on a virtual reality headset and turn it on. Immediately you find yourself standing in front of a table. On the table sits an apple.
Imagine you bend down and inspect the apple from multiple angles. You see the light shimmering off, and it looks tasty. Imagine you are wearing gloves that allow you to pick up the apple. You reach down and grab the apple and lift it off the table and toss it across the room.
Does the apple you see in your headset physically exist? Is there physical matter there making up the apple you are holding? No. In VR, you are simply looking at a screen, and although you are seeing and interacting with many things that seem real, its really just images on a screen. Although the VR apple appears solid, it is not. Its virtual.
There is no difference between that example, and the sight you experience every day. Just because you see, taste, and smell an apple in your fridge, does not mean there is a physical entity that exists outside of the realm of your senses. The illusion of "physical" reality is brought about by our religious devotion to the idea that something exists other than the contents of our minds. Even though we never have, nor never will experience anything other than the 5 senses and thought, people are still trapped by the belief that there is something else out there.
I originally thought this is related to physics, where things that we can experience are made of atoms which are mostly empty space. However, Buddhism is almost always related to the mind. So, how does this apply to the mind?
Physics is a useful conceptual framework for accomplishing a task. Just because it is useful for something, does not mean it reflects the nature of reality or even exists. Many amazing things have come about due to an understanding of this very deep and amazing conceptual framework that has been developed. Although it is useful, does that mean it is accurate in describing our universe?
It can be a hard pill to swallow that all of physics may not describe anything real, so lets give a smaller example that is more manageable, but the concept still applies to all of physics. Think about the number two. Mathematics wouldn't be the same without two. Think of all the amazing uses we have for two. Although the number two is extremely useful, does two even exist in reality? Can you find a two and save it for later? Does it describe anything real? Or is it simply a concept we have constructed as a useful tool?
When exploring reality, it is important to be meticulous and question everything you believe to be true. Even something everyone in America is taught to be "true" like physics should be scrutinized. All you have ever experienced is the mind. On what basis do you believe something exists other than the mind? Beginning to follow this movement inward is paramount to understanding and experiencing the freedom that is enlightenment.
I hope what I wrote will be helpful to you in some way. I pray for nothing more than your liberation this lifetime.