As per Branch support: The page being referred to is called a Deepview. A Deepview is a mobile web splash page, hosted by Branch, that gives a preview of the in-app content behind a given Branch link.
For more information on the same, you can go through the documentation below:
https://help.branch.io/using-branch/docs/deepviews#overview
You can navigate to your Deepview setting from the Configuration page and then navigate to the Deepviews tab. You can not edit the existing Deepviews, however, you can create your own custom Deepview. Do go through the following documentation for a step by step guide:
https://help.branch.io/using-branch/docs/deepviews#1-enable-default-deepviews-templates
https://help.branch.io/using-branch/docs/deepviews#1a-create-a-custom-deepview-template