The general design is this: there's a code unit (.pas) and declarative UI property file (.dfm) for every form in your application. When you modify any published properties for any controls/components on the form via the object inspector, these modifications are stored in the .dfm file. Any changes to the controls/components on the form (adding a control, removing a control, renaming a control) are immediately reflected in the .pas code for the form. And, the code is not just generated code that you can't touch, it is code that you are modifying along with the IDE. You can jump back and forth between the UI designer and code editor with a simple F12 key press.
When an application is compiled, the compiler automatically bundles the .dfm files as resources in the resultant binary and streams the properties in during form instantiation.
Where this all really shines is with event handling. You can, for example, double-click a button on the form and the designer will drop you into the code editor with a fully-defined OnClick event handler all ready to go. This is because Delphi has the concept of default events for controls/components. For non-default events, you can do the same by double-clicking on the event name in the object inspector.
The other thing is that the UI designer is running real code in the background for controls/components, so the design-time environment mirrors the run-time environment, and control/component designers can interrogate the RTL to find out which state they are in, and act accordingly. You can also write custom property editors that allow for complex property editing in a nice interface.
Our product, Elevate Web Builder, works exactly like this for building single-page web applications. It's extremely productive and a very good fit for web applications because, unlike desktop applications, there's not as much risk of mixing back-end code into your UI code and starting to makes things messy.
The key is that you need a strongly-typed language to make this all happen, so I'm not sure how applicable any of this is to Python.
That said, especially the custom property editors would make e.g. Xcode’s interface builder almost worth looking at. I’ve just given up on graphical designers at all—last I checked thy were all terrible with responsive design, which is ironic on an ide that claims to target a mobile platform.
As for responsive design in graphical editors: what part do you find that they are terrible at, or is it that they don't do it at all ? Our previously-mentioned Elevate Web Builder will allow the developer to design responsive layouts that flow and move as you resize the container in the designer so that you can see how the design will behave at run-time.