Authentication Manager is one of the key capabilities from PnP core component and it provides the methods to authenticate different SharePoint environments (SharePoint Online, SharePoint 2013, SharePoint 2016) irrespective of any authentication methods configured to the SharePoint sites.

The methods used for authentication are available under OfficeDevPnP.Core.AuthenticationManager class from OfficeDevPnP.Core assembly. I have listed those methods based on the environment type.

SharePoint Online

SharePoint On-Premises

SharePoint Online & On-Premises