Apex - get values in nested JSON

Use serialization/deserialization and it's quite easy. You can generate the definitions using JSON2Apex, but it's not rocket surgery and I typically prefer to do it by hand. For example you might do:

public class MyPayload
{
    public class Result
    {
        public final String uniqueId;
        public final IdInformation idInformation;
        public final BioInformation bioInformation;
    }
    public class BioInformation
    {
        final Boolean confidInd;
        final String ssn, sex, userId;
    }
    public class IdInformation
    {
        final String changeIndicator, dataOrigin, entityIndicator, mID, rowId, userId;
        final String firstName, lastName, middleName, surnamePrefix;
    }
}

Then it's easy to work with the results.

List<MyPayload.Result> results = (List<MyPayload.Result>)JSON.deserialize(
    somePayload, List<MyPayload.Result>.class
);
for (MyPayload.Result result : results)
{
    system.debug(result.idInformation.firstName);
}