I apologize if my choice of words is poor or this is a duplicate post, but I don’t know the exact term for what I want to ask, unfortunately. However, I hope you can understand my question with the help of the screenshots.

Basically, I have noticed that most of my UI elements are fixed, and that is probably a good design choice, but in some cases, I want them to expand when the screen size increases, either to fullscreen mode or maximized window mode.

Here is an image that illustrates my problem by using the Godot Editor:

My main question is, how do I go about this?
Thanks in advance!

If you click on the control in the scene there is a container sizing property on the right hand side.

If you define your interface layout using anchor points and containers they should always keep intended layout.

As an example lets assume you want to have rect always hiding left half of the window independent on window size. You can achieve that by:

HBoxContainer (anchor full rect)
    - ColorRect (container sizing expand)
    - Control (container sizing expand)

In such case HBoxContainer should be always full window size, ColorRect and Control would take up half of the available space, so they both should be half of the window.