What are symmetric keys and their advantages or disadvantages?

Symmetric keys use the same key for encryption and decryption

When creating an instance what kind of key are we creating?

(public and private keys) for secure remote access.

What is the disadvantage of AWS managed keys?

AWS managed keys offer limited control, as AWS handles key rotation, deletion, and policy management instead of you.

What is the advantage of customer managed keys?

Customer managed keys give you full control over key creation, rotation, permissions

What kind of AWS KMS can be used when customer wants to encrypt the data, but does not want to manage the keys and not pay for it?

Use the AWS-owned CMK in KMS, as it encrypts data without customer key management or extra cost.