Answering the first part of your question:

resource "aws_opensearchserverless_collection" "collection" {
  name = "bedrock-kb-collection"
  type = "VECTORSEARCH"
  description = "Vector collection for Bedrock knowledge base"

  depends_on = [aws_opensearchserverless_security_policy.encryption, aws_opensearchserverless_security_policy.network]
}

Yes, you can point your bedrock knowledge base to s3 bucket by setting up the aws_bedrockagent_knowledge_base & aws_bedrockagent_data_source.

Ref links: https://registry.terraform.io/providers/hashicorp/aws/5.95.0/docs/resources/opensearchserverless_collection
https://registry.terraform.io/providers/hashicorp/aws/5.95.0/docs/resources/bedrockagent_knowledge_base
https://registry.terraform.io/providers/hashicorp/aws/5.95.0/docs/resources/bedrockagent_data_source