Power Apps Portal
Power Apps portals to create external-facing websites that allow users outside their organizations to sign in with a wide variety of identities, create and view data in Dataverse.
Requirement: Power apps portal website default Landing Page is the home page which is accessible to everyone. Only signed in users should see the contents of the website. So, we replace the default landing page with sign in page.
Step 1: Create a power apps portal and open it in the editor (If you already have a portal you can use that).
Step 2: Make sure that the home page is selected for editing in the portal editor.
Step 3: Select the permissions settings showed on the righthand side of the editor. And deselect the option – page available to everyone.
Step 4: After deselecting the page available to everyone option you can select the user roles who could access the home page. Select the Authenticated users from the roles (You can also use your custom roles).
Step 5: After all the above mentioned steps save the changes. By selecting the authenticated user role now if a new user tries to access the site (any page), they will be redirected to the sign in page.
Hope this helps!