Active Jobs, at least how implemented in 4.2 is not its own background processor. Its just a common api for using the background processor of your choice (Sidekiq, Resque, Delayed job etc).
ActievJob actually does default to :inline, which uses the Rails process to execute the job, so in a way, it kind of is it's own background processor, but you can easily swap out, as it implements an API for creating and scheduling jobs.