Default front page not showing https My website URL shows in browser https

But on https://test.com/admin/config/system/site-information



It's showing http.

What will be cause of this? Server configuration (apache)?

Does it relate to Base URL?
Yes. The field prefix is populated by the RequestContext:: getCompleteBaseUrl() method, the underlying property, completeBaseUrl, is assigned from $GLOBALS['base_url'] if it's set. 

You'll have to test that out to see if that's the case for you, because you might have a web stack configuration that causes Drupal only to see unencrypted requests (e.g. HTTPS only handled at your load balancer / proxy server level), thus displaying the HTTP base.