Hi Rohit, I split this question to a separate thread.
A key thing to keep in mind is that authentication is always done by a third party identity provider. Qlik Sense does not authenticate the accessing user.
In your case the external users can not access Qlik Sense, because the authentication configuration in the accesses virtual proxy can not identity the accessing user. You need to define a virtual proxy for your external users, which can redirect authentication of your external users to an identity provider that contains the external users.
UDC only synchronizes user meta data from a directory into Qlik Sense. It does not directly affect user access to Qlik Sense. The meta data can be utilized in security rules to govern user access.
Take a look at https://www.qlik.com/us/-/media/files/resource-library/global-us/register/whitepapers/wp-qlik-sense-security-overview.pd… and on page 6 in "Qlik Sense Security User Access Workflow" you can see an overview of the user access process.