authentication – SharePoint online authantication when using SharePointOnlineCredentials

For how this will work, you will register an Azure AD app with the appropriate Graph API permissions (there is also the v1 API which uses Client ID/Secret, registered in SharePoint itself).

See Using CSOM for .NET Standard instead of CSOM for .NET Framework.

See SharePoint authentication for how SharePoint Online auth is performed. Essentially, you’re issued a FedAuth cookie.