Hello @Timothy A. Dilbert ,
Thanks for posting your query.
If your organization uses a third-party federation solution, you can configure single sign-on for your on-premises Active Directory users with Microsoft Online services, such as Microsoft 365, provided the third-party federation solution is compatible with Azure Active Directory.
For questions regarding compatibility, please contact your identity provider. If you would like to see a list of identity providers who have previously been tested for compatibility with Azure AD, by Microsoft, see Azure AD identity provider compatibility docs.
This depends on the offering and compatibility of your IDP. Based upon this question for OKTA as per https://help.okta.com/en-us/Content/Topics/Apps/Office365-Deployment/add-o365.htm, you need to define both tenant to which IDP will be connected and domain you want to federate from the tenant. So, users belonging to non-federated domain will be using Azure AD as IDP
Please do let me know if you have any further queries in the comments section.
Thanks,
Akshay Kaushik
Please "Accept the answer", "Upvote" and rate your experience if the suggestion works as per your business need. This will help us and others in the community as well.