Hi @Rahul18081 
This can be found easily in BOL. Only certain ports/URLs are required:

https://documentation.commvault.com/11.26/expert/103533_exchange_mailbox_agent_user_mailbox_exchange_online_through_on_premises_active_directory_port_requirements.html

Yes if you are protecting mailboxes in O365 that proxy must have access to O365. Inbound and Outbound.

@Scott Reynolds is below correct

Communication with Office 365 should be both ways(inbound ad outbound) because when backup runs it downloads data from Office 365 onto the Access node and the onto the s3 bucket. During restore it uploads the data from s3 bucket to Access Node and then onto the Office 365.

@Rahul18081  Assuming your disk/backup target is S3 bucket then yes that is brief summary or flow.