Can someone please explain how open2study.com is maintaining a common login between moodle and drupal .
Thank you.
You've asked this question before. Maybe you could try asking a more specific question?
As mentioned in the reply to your other post, the open2study site uses SAML based authentication. There are various plugins that support SAML for Moodle and Drupal.
using SAML does require some advanced knowledge so if you are struggling you could pay a Moodle Partner to help you with the process.
I work for the NZ Moodle Partner Catalyst IT (our AU office looks after the open2study site).
SAML is frequently implemented using a technology referred to as Shibboleth, that is worth looking up in your research. It is not trivial to implement so experts like Catalyst are worth consulting.
https://docs.moodle.org/dev/Core_APIs and https://docs.moodle.org/dev/Plugins are probably good starting points if you are looking for general information. And other pages in that dev docs.