I'm mostly a Front-end Developer. One day I heard about such a thing at work like "authentication service", "registration service" and it turned out our product's backend has many services running apparently separately from each other. I tried making some simple backend for my own projects but it's always been like I have only one server running that backend app, all requests are coming to one place and the backend app doesn't request anything but database data on its part. Where could I read more about separating backend to many services and about cons and pros of this approach?
The term that you're likely looking for is microservice. This is the practice of running multiple single purpose or narrowly scoped service.
It's also possible that your backend team used the term "service" to refer to different endpoints of a monolithic service. Even with microservice architecture, it's rather uncommon to split registration service from authentication service, as they both usually would come under the user management service.