if entered aws --version cmd in my powershell but cmdlet is invalid. why and what i have to install?

AWS CLI is not installed , you should install it

how could you able to connect to an instance private subnet which dont have a public IP to the internet?

you can have have jump server or nat gateway

without NAT gateway is there any possiblity to connect a private ec2 without a public IP to access data in s3 bucket?|

you can use VPC gateway endpoint for S3

which act like a failover cluster ZRS? or GZRS? how?

ZRS acts like a failover cluster by keeping your data synced across multiple zones in one region for automatic zone-level protection, while GZRS adds protection by also copying data to a second region to handle full regional failures.

What is the format of ARN for an AWS service?

ARN-Amazon Resourve Name is a Unique identifier assigned for each object/resources in AWS