1. Because that's what Microsoft decided, note this is for the Power Apps per App licence, if you have a higher level licence this is not the case. (If you have a higher licence, you can have more than one Portal per instance, however they usually have to be of different types e.g. Custom + Customer Self-Service).
2. There is integration with SharePoint but only in so far as pointing to a folder (document location), if you need more you would need to work out how to get the info you need into Dataverse/CDS.
3. Maybe, depends on what capabilities you need, It is not an app and as such needs a supported browser on the device it will be used from, it can be used for both internal (Azure AD) and external users.
4. Azure AD is setup by default, you have your internal users use the Azure AD button on the login page. Note, there are various additional settings such as remember me, session timeouts etc that can also be set.