The website for a portal app is generated from the configuration data held in entities in the CDS. You can add a page to the website by simply creating a record in the Web Pages entity in Portal Management app. However, it is easier to use the Power Apps portals Studio to add pages and change the website navigation.
You open Power Apps portals Studio by clicking on the ellipses (…) next to the portal app and selecting Edit. Power Apps portals Studio will open, as shown in the following screenshot:
Power Apps portals Studio is a WYSIWYG visual editor for customizing the portal website by adding and configuring components on the web pages. To add a web page, click on + New page, as shown in the preceding screenshot. You then select a web page template that controls the basic layout of the page.