You don't create services. You create classes and they are already SERVICES if they DO something.
So, the answer to the first question is simple: in reality, you don't need to do anything special to turn an object into a service. It needs only one aspect to be qualified as a service. It simply needs to do something. In other words, if a class contains methods that actually perform a task, you can call the object instantiated from that class a service. For example, Symfony entities are not services because they normally consist of a bunch of property definitions. However, any PHP object that takes these entities and performs some action on them can be called a service.
So, whenever you create a new class, you are potentially creating a new service. Service is just a fancy new name. That's all.