why we create snapshot as a volume?

Because Snapshots are not Active Storage and to access data mountable file system is needed.

what is snapshot?

Snapshot is a backup to the EBS volume of EC2 instance.

what is the process of restoring the snapshot in another instance?

In snapshots - Select snapshot - in Actions - create volume from snapshot
Select AZ - us-east-1b - create volume

if the all AV is shutdown then how we restore the files?

Even if all EC2 instances are shut down or terminated files can be restored from EBS snapshots of the volumes where the files were stored

If we create multiple snapshot how we restore the file ?

1. identify the snapshot that contains the specific version of the files you need to restore,
2. Create a New Volume from the Snapshot,
3. Attach the New Volume to an EC2 Instance,
4. Connect to the EC2 Instance and Mount the Volume