We need to authenticate users to Salesforce Community using internal app login which cannot support SAML or OpenID. Is there any other solution available? Is "custom authentication provider" the only way to accomplish this requirement? Is "delegated authentication" only applicable for the users who are in Active directory?
If external community users are not being authenticated by SAML or OpenID Connect then only option to use custom authentication.
An authentication provider lets your users log in to your Salesforce org using their login credentials from an external service provider. Salesforce provides authentication providers for apps that support the OpenID Connect protocol, such as Google, Facebook, Twitter, and LinkedIn.
For apps that don’t support OpenID Connect, Salesforce provides an Apex Auth.AuthProviderPluginClass abstract class to create a custom authentication provider.
Is "delegated authentication" only applicable for the users who are in Active directory?
No, it can also be applied for other authentication provider like
PingFederate. Here is an example to setup for that.