Hi there,
looks like you installed the Vinyasa Site which uses the Header Element:
https://docs.generatepress.com/article/header-element-overview/
One of the Options of the Header Element is to use the Featured Image as the Background.
Which is being used by the Blog Posts Header element the site installed ( found in Appearance > Elements ).
So for Pages – you can create a new Header element with the same settings as those already added and set its Display Rules to Pages > All Pages