There is a notion of Section Layouts whether in the Pages word-processing or page layout mode. One image (or shape) can be placed in the background below the text and other objects on each page of a section, or as required, the entire document beyond the first section where the section layout resides.
Add watermarks and background objects in Pages on Mac - Apple Support
In your example image, you could select that orange square, change its location, size, opacity, and then Arrange menu > Section Layouts > Move Objects to Section Layout. That places it below everything just like a watermark.