For you to understand the services clearly, you need to understand the same from OSGI perspective. It is basically object instance registered in Felix Console. I think it would be better for you to read some documentation around the same to start with . https://osgi.org/download/osgi.core-7.0.0-early-draft-2017-10.pdf This is a draft version I took from the OSGI website, and page 133 specifies you what exactly a service is in OSGI perspective. That said, for you to register any class as service you need to have atleast 1 interface.
Once you have the interface/s ready , you can implement those to a service using @Service (this is now an old approach using Apache Felix Maven SCR ) Going forward , we need to use OSGI annotations according to OSGI R6 specifications , using which you need to use the below syntax to implement any interface as service