function updateIdToDelete(id) {
component = Qt.createComponent("main.qml");
sprite = component.createObject(mainWindow, {idToDelete: id});

you are creating your main.qml ( ApplicationWindow ) again with this code, so it is normal that it opens a new window

you are trying to pass a value from InitialScreen Component to your main.qml right?
How/where is your InitialScreen created ? If it is in you main.qml then you can simple do like this

//main.qml

ApplicationWindow {
   property int idToDelete : initialScreen.idToDelete
  
  InitialScreen{
   id: initialScreen
  }
}

I was sortof thinking of this solution too, but I was using it like this:

ApplicationWindow {

    id: mainWindow
    StackView {
        id: stackView
        anchors.fill: parent
        initialItem: initialScreen
    }

    Component {
        id: initialScreen
        InitialScreen {}
    }
}

and this way initialScreen.idToDelete was not reachable.
However redesigning code to what you suggested solved the problem :)

Thank you a lot!

function updateIdToDelete(id) {
component = Qt.createComponent("main.qml");
sprite = component.createObject(mainWindow, {idToDelete: id});

you are creating your main.qml ( ApplicationWindow ) again with this code, so it is normal that it opens a new window

you are trying to pass a value from InitialScreen Component to your main.qml right?
How/where is your InitialScreen created ? If it is in you main.qml then you can simple do like this

//main.qml

ApplicationWindow {
   property int idToDelete : initialScreen.idToDelete
  
  InitialScreen{
   id: initialScreen
  }
}