Those are very good questions. Here is some information on those metrics.
Background threads: these threads complete tasks that are not related to individual requests made to the CPU.
Garbage collection CPU: this is the time the CPU spends in suspension while it cleans up threads and references to objects that are no longer in use.
Service request CPU: this is time the CPU spends completing requests made to it by services on the host.
Also, here are some Dynatrace blogs that go more in depth with those topics.