I have already run the PowerShell command Connect-SPFarmToAAD on the on-premises farm. (Connect-SPFarmToAAD is document here in detail: https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/use-an-office-365-sharepoint-site-to-authorize-provider-hosted-add-ins-on-an-on.)
When registering a new app on the appregnew.aspx page of my on-premises farm. I am not seeing it in my Azure AD environment anywhere. Should this work on-premises? Is there some other configuration to the farm that must be done?
I found a manual workaround that I could make a new client id in Azure AD via Azure app registration and give it a secret. Then create that same client id app on the SharePoint side via appregnew.aspx. Finally grant it permissions in appinv.aspx to the site or web I was working on. I didn't use the same secret which didn't matter actually, just as long as the ID's are the same things worked great for the standard ACS token request process.
When doing the appregnew.aspx process on a SharePoint Online I am seeing a new service principal created in Azure AD and things are working as expected.