how to show the json data based on the dropdown selection in flutter?

You can copy paste run full code below
You can compare selected dropdown value and show only specific data
and return Container() (don't display) for others
code snippet

ListView.builder(
      itemCount: payload == null ? 0 : payload.data.length,
      shrinkWrap: true,
      itemBuilder: (context, index) {
        return ListView.builder(
            shrinkWrap: true,
            physics: NeverScrollableScrollPhysics(),
            itemCount: payload == null
                ? 0
                : payload.data[index].details.length,
            itemBuilder: (context, i) {
              if (dropdownValue == null) {
                return ListTile(
                    title: Text(
                        "${payload.data[index].details[i].id}"));
              } else {
                if (payload.data[index] == dropdownValue) {
                  return ListTile(
                      title: Text(
                          "${payload.data[index].details[i].id}"));
                } else {
                  return Container();
                }
              }
            });
      }),

working demo

full code

import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
// To parse this JSON data, do
//
//     final payload = payloadFromJson(jsonString);

import 'dart:convert';

Payload payloadFromJson(String str) => Payload.fromJson(json.decode(str));

String payloadToJson(Payload data) => json.encode(data.toJson());

class Payload {
  List<Datum> data;

  Payload({
    this.data,
  });

  factory Payload.fromJson(Map<String, dynamic> json) => Payload(
        data: List<Datum>.from(json["data"].map((x) => Datum.fromJson(x))),
      );

  Map<String, dynamic> toJson() => {
        "data": List<dynamic>.from(data.map((x) => x.toJson())),
      };
}

class Datum {
  DateTime date;
  List<Detail> details;

  Datum({
    this.date,
    this.details,
  });

  factory Datum.fromJson(Map<String, dynamic> json) => Datum(
        date: DateTime.parse(json["Date"]),
        details:
            List<Detail>.from(json["Details"].map((x) => Detail.fromJson(x))),
      );

  Map<String, dynamic> toJson() => {
        "Date":
            "${date.year.toString().padLeft(4, '0')}-${date.month.toString().padLeft(2, '0')}-${date.day.toString().padLeft(2, '0')}",
        "Details": List<dynamic>.from(details.map((x) => x.toJson())),
      };
}

class Detail {
  int id;

  Detail({
    this.id,
  });

  factory Detail.fromJson(Map<String, dynamic> json) => Detail(
        id: json["id"],
      );

  Map<String, dynamic> toJson() => {
        "id": id,
      };
}

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  Payload payload;
  Datum dropdownValue = null;

  Future getData() async {
    var response = await http
        .get("https://www.json-generator.com/api/json/get/bOpGzigKOG?indent=2");
    payload = payloadFromJson(response.body);

    setState(() {});
    return 'success';
  }

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback((_) {
      getData();
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: payload == null
          ? CircularProgressIndicator()
          : Center(
              child: Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  Center(
                    child: DropdownButton<Datum>(
                      //isDense: true,
                      hint: Text('Choose'),
                      value: dropdownValue,
                      icon: Icon(Icons.check_circle_outline),
                      iconSize: 24,
                      elevation: 16,
                      style: TextStyle(color: Colors.deepPurple),
                      underline: Container(
                        height: 2,
                        color: Colors.blue[300],
                      ),
                      onChanged: (Datum newValue) {
                        setState(() {
                          dropdownValue = newValue;
                        });
                      },
                      items: payload.data
                          .map<DropdownMenuItem<Datum>>((Datum value) {
                        return DropdownMenuItem<Datum>(
                            value: value,
                            child: Text(
                                ' ${value.date.year.toString()}/${value.date.month.toString()}/${value.date.day.toString()}'));
                      }).toList(),
                    ),
                  ),
                  ListView.builder(
                      itemCount: payload == null ? 0 : payload.data.length,
                      shrinkWrap: true,
                      itemBuilder: (context, index) {
                        return ListView.builder(
                            shrinkWrap: true,
                            physics: NeverScrollableScrollPhysics(),
                            itemCount: payload == null
                                ? 0
                                : payload.data[index].details.length,
                            itemBuilder: (context, i) {
                              if (dropdownValue == null) {
                                return ListTile(
                                    title: Text(
                                        "${payload.data[index].details[i].id}"));
                              } else {
                                if (payload.data[index] == dropdownValue) {
                                  return ListTile(
                                      title: Text(
                                          "${payload.data[index].details[i].id}"));
                                } else {
                                  return Container();
                                }
                              }
                            });
                      }),
                  Text(
                    'You have pushed the button this many times:',
                  ),
                  Text(
                    '$_counter',
                    style: Theme.of(context).textTheme.headline4,
                  ),
                ],
              ),
            ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}