PPowerApps Portals mainly used for engaging with Customers and Partners.
PowerApps Portals allow organizations to create websites which can be shared with users external to their organization either anonymously or through the login provider of their choice like LinkedIn, Microsoft Account, other commercial login providers.
You can refer below links for more info.
You can view videos from below playlist:
Also, You can refer below blogs
Also @OliverRodrigues blog
You can use PowerApps Portal with Dynamics 365. PowerApps Portal needs some Data Source for data. So you can use it with Dynamics 365.