How can we add/create objects in the signin page? Like texts, boxes, buttons etc..

[Robert94]

I`ve read this blog post Customize the sign-in and registration page in Power App Portals - Ulrikke Akerbæk (akerbak.com)
It says there how can we customise the signin page, but it doesn`t say how do we actually add elements in there, that later can be customised.