Are you trying to connect to an EC2 instance? If so, this: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AccessingInstancesLinux.html AWS document should help, specifically:
For more information about how to find the private key, the user name for your instance, and the DNS name or IPv6 address for an instance, see Locate the private key and set the permissions and Get information about your instance.
(Edited to add links back into the quote)
You tagged this as "Database" - are you migrating a database hosted on EC2 or on RDS?
If this is EC2, you'll be able to find the hostname in the AWS EC2 console, the default port will be 22 (if it wasn't changed afterwards), the username will depend on the OS you're running (for example it's "ec2-user" for Amazon Linux).
The private key and its passphrase aren't stored anywhere in AWS, you'll need to find out which one was used on your side and what the passphrase was. If the existing private key isn't available anymore, you can follow this document to recover the instance access: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/TroubleshootingInstancesConnecting.html#replacing-lost-key-pair
If this is RDS, then you don't typically have any SSH access to it.
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