What is AWS CLI

AWS CLI ( command line interface) is a tool that allows to interact with AWS from the command line. It enables you to manage AWS services without using the AWS Management Console.

What is AWS CLI

How to migrate data across AWS accounts, AWS Regions, or Availability Zones?

what is SSH and where it is used?

SSH ( Secure Shell) is a network protocol used to securely connect to remote servers over the internet.

Is it possible to assign one EBS volume to two different instances at the same time in the same AZ.?

Yes, it is possible. But it requires a feature known as "multi-attach" to be enabled.