All the managers(will be covered below) defined as Symfony services. In a nutshell, a service is a class which implement a logic and can receive other services as an argument and can be injected to other services as well. For more info go to the DependencyInjection Component documentation.
You can add as many services as you'd like to. As always, let's have a look at
the hooks.yml file:
services:
- 'services.local.yml'
By default, you can add services in the services.local.yml but if you want to
manage that in other files you can add them in the hooks.local.yml file.