Can any please help to understand data transfer charges ? by which metric and service i can check, how much data is transferred.?

In CloudFront's CloudWatch metrics, "BytesDownloaded" and "BytesUploaded" are metrics related to data transfer.
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/programming-cloudwatch-metrics.html

I thought it would be possible to check the "sc-bytes" by enabling the CloudFront access log.
https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/AccessLogs.html#LogFileFormat

The total number of bytes that the server sent to the viewer in response to the request, including headers. For WebSocket connections, this is the total number of bytes sent from the server to the client through the connection.