When activating a Lightning page from the Lightning App Builder, three options are available:

Assign the page to a combination of apps and profiles. This option allows users to specify which apps and profiles will see the custom Lightning page instead of the default page.

Make the page the org default. This option makes the custom Lightning page the default for all users in the org regardless of their app or profile.

Make the page the default homepage for specific apps. This option allows users to set different homepages for different apps in Lightning Experience. [Source]