Is there any list of APIs that I can refer for reading and wirting of the Metadata ?

Hi ​@Apurva Kapoor , 

Thank you for your question!

As a starting point, I would recommend referring to our documentation. It also contains a list of commonly used GraphQL operations - https://docs.ataccama.com/one/latest/one-apis/one-api.html#graphql-operations. 
Also, I recommend going though this Guide ‘How to run GrapQL requests’, it gives you tips how to explore MMM and suggests Web Extensions that will help you with your GraphQL requests.

Hope it helps! Please let me know if you have any questions.

Kind regards,
Ekaterina