What are the CI4 ways to use Database and Library in a Helper?

Again I am little bit confused about Service. I have read through the docs but still not clear about understanding "What is service?".

In CIv4 a Service is a type of factory to create new instances of the desired class. In other words, think of it as a kind of "helper" to load and instantiate a class. If you haven't already, read the v4 Documentation - Services info here