I am planning to make a simple program, and consider to make it in Qt. I read that programs made in Qt could be run in different operating systems. Does it mean that when I create a desktop application for Windows, it would be executable in a smartphone? I just doesn't fit in my mind. For example, I don't have iOS experience, but as far as I know you need to have a Mac computer or at least to make specific configuration to Windows computer in order to develop applications. Now, if I use Qt in standard Windows(no configs.), would that program really run on iOS and could it be transferred to Appstore just with the payment without any modifications? I have always worked just in Windows, and mainly in Java. The "multiplatform" concept made me consider to start development in Qt, but I am not sure because I don't understand it well. I would appreciate if you could clarify.
I read that programs made in Qt could be run in different operating systems
No. You have to re-compile your application on each target system.
Java is different. Java code is compiled into a special binary that is run by Java Virtual Machine (JVM). Because JVM is multi-platform you can usually run a java application in any platform you have JVM available.
When you compile a Qt application which is most likely written in C++, it's linked to OS libraries and as a result that application will only run on that OS.
Now, Qt is a multi-platform SDK, the code you write using Qt can be compiled for all the platforms that Qt supports and results will be same or similar. I say similar because, for example Qt tries to emulate native widgets of the operation system. You will have to test and maybe tweak your application according to target OS. Mobile user interfaces are very different than desktop, so you may have to re-implement GUI of your application separately for mobile.