Containers: container.id from the APM metrics data streams.
Hosts: If the application is containerized (if the APM metrics documents include container.id), host.name is used from the infrastructure data streams (filtered by container.id). If not, host.hostname is used from the APM metrics data streams.
Really useful information. So one thing, for my non-containerized app I am not seeing any data in the infrastructure tab. Interestingly the host.hostname field for the APM data stream is lowercase. It is uppercase for the infrastructure data. Host name is the same!