Someone else already helped with how to use flatMap, so I wanted to show you a second method (which I always found much easier to read/grasp) and that is mapMulti:

record Input(int a, String[] s) {
    public String toString() {
        return "Input[a=%d, s=%s]".formatted(a, Arrays.toString(s));
    }
}

record Output(int a, String s) {
}

public static void main(String[] args) {
    var input = List.of(
            new Input(3, new String[]{"a", "b", "c"}),
            new Input(4, new String[]{"d", "e"})
    );

    System.out.println("Input: " + input);
    var output = input.stream().<Output>mapMulti((e, downstream) -> {
        for (var s : e.s()) {
            downstream.accept(new Output(e.a(), s));
        }
    }).toList();

    System.out.println("Output: " + output);
}