Zones in the layout builder can be layered, meaning whatever is towards the top of the list sits on top. This can be very useful for using image overlays on zones and general layout design.
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To change the order (hierarchy) of zones on a page, click and hold the relevant zone you want to move in the main layout menu (left hand side on desktop), drag it to the new desired new order / layer and then release.

Further Reading: Read here all other Zone Articles.