How methods of stream interface work in java?

Recently I was learning about Java 8 features so I started with a lambda expression, then I came across the Java stream API, now I am trying to wrap my head around the methods in stream API, How they work?

What I understood about lambda expression is if we want to pass the lambda expression to the method then we need to create an interface with the single unimplemented method (Java 8 interface can have default implementation) that matches the signature of a lambda expression. then the method to which the lambda expression is passed can execute the lambda expression by calling the interface method. So I can say that a lambda expression works as an implementation of the interface method.

But when I saw Stream API it has some methods which change the stream, for example below code just print integers by filtering integers which are less than 5.

import java.util.Arrays;
import java.util.List;


public class StreamDemo {
   static List<Integer> list= Arrays.asList(new Integer(1), new Integer(2), new Integer(3), new Integer(4), new Integer(5), new Integer(6));
    public static void main(String[] ar) {
        list.parallelStream()
                .filter( x-> x<5)
                .forEach(System.out::print);
    }
}

But I am not understanding, how filter work as it does not have an implementation, I tried to see the implementation from Intellij which takes me to the Stream interface where the filter is non-implemented method which takes the parameter Predicate reference, but there is no method call of predicate method boolean test(T t); so how come Java evaluate the result, similarly there is method distinct which does not have an implementation but still gives me accurate result.

Thanks in advance.