The classic agent is for the AppMon and nowadays with the latest AppMon platform we have another agent called AppMon agent and these agents are technology based. AppMon provides the ability to choose between 2 different agent platforms: the Classic Agent, which is the default agent platform and the new AppMon Agent.
OneAgent is for the Dynatrace platform and is responsible for collecting all monitoring data within your monitored environment. Only a single OneAgent per host is required to collect all relevant monitoring data—even if your hosts are deployed within Docker containers, microservices architectures, or cloud-based infrastructure.
You use PaaS agents in cases where you don't directly control the hosts, for example those that run in Azure or Cloud Foundry, or in other PaaS environments.
If you'd like to learn more about OneAgent, its installation, and real-life examples, I recommend watching a recording of the following webinar: What is Dynatrace and how to get started