I am planning to use opensearch hosted by AWS. I found there are two libraries @aws-sdk/client-opensearch and @opensearch-project/opensearch. I wonder which one I should use. Are there meant to be very different?
They're both useful for different things.
Most likely you're interested in the actual opensearch features, like adding items to the index, updating the index, searching against the index. In this case you'll find @opensearch-project/opensearch useful. These AWS docs are helpful if this is what you're digging into.
If you're looking to get information about the deployed domain, and to manage it, then @aws-sdk/client-opensearch may be useful.