In Qtcreator, how to center-align my layout?

Question

I'm a beginner with Qt Creator. I can't figure out how to center my layout in the window, even when I expand it to full screen size.

Please review my object trees and GUI image. After running the code, the layout appears centered, but when the window is maximized, it shifts to the left.

(Apologies for the long image; I have a wide monitor)

1. Captured GUI, object trees
2. Before fullscreen
3. After fullscreen

I googled, asked to GPT but I couldn't find a way.