By default your EC2 instances do not upload logs to CloudWatch Logs, they need to have a CloudWatch Logs agent installed with the proper IAM permissions in order to have logs sent to CloudWatch Logs. A benefit to this is you can set up this CloudWatch Logs Agent on your on-prem infrastructure as well to have logs sent to CloudWatch Logs from your on-prem server.
You have both the CloudWatch Logs Agent and CloudWatch Unified Agent. The Logs Agent is an older version and can only send data to CloudWatch Logs. The Cloud Unified Agent allows for centralized management of all Unified Agents via the SSM Parameter store, and more granular metrics, which the Logs Agent does not perform. These can range from CPU, memory, RAM, Disk, Netstat active TCP/UDP connections or net packets, etc. The data can then be sent to CloudWatch logs for further analysis. In short, the Unified Agent offers granularity that the Logs Agent can’t provide, the Logs Agent can’t even perform metrics at all on its own. Both are for use of EC2 instances and on-prem servers.