Firstly, sorry for using non-professional terms, I'm a beginner.
I tried using the union boolean on two mesh objects, and it worked properly. Then, I had to remove the boolean, but even though I haven't applied it yet, the red object disappered, and the orange didn't. It doesn't return when I'm pressing the eye and camera icons next to its name, and when I enter edit mode it still appears.
I pressed Ctrl+Shift+B to open the Bool Tool screen, and chose Brush Boolean - Difference. Afterwards, The boolean was added to the screen, and the small cube disappeared.
Then, I removed the modifier but the small cube stayed invisible except for its outline. The camera icon next to its name was off, so I pressed on it but nothing changed.
Here is how it looks in Edit Mode: