How to access list of objects inside a list of Objects in Dart

Quick answer

You can nest two mapping of your JSON data:

listOfItem.map((category) => Column(
  children: (category['restaurants'] as List<Map<String, dynamic>>)
      .map((restaurant) => Text(restaurant['name'])).toList()
).toList();

Remember to use toList() as a map operation does not return a List but an Iterable.

Basic Full Solution

Here is a first solution based on your data and the Flutter Tabs.

Full source code:

import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      home: HomePage(),
    ),
  );
}

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      length: data.length,
      child: Scaffold(
        appBar: AppBar(
          title: Text('Restaurants'),
          bottom: TabBar(
            tabs: data.map((category) => Tab(text: category['types'])).toList(),
          ),
        ),
        body: TabBarView(
          children: data
              .map((category) => ListView(
                    children:
                        (category['restaurants'] as List<Map<String, dynamic>>)
                            .map(
                              (restaurant) => ListTile(
                                title: Text(restaurant['name']),
                                subtitle: Text(restaurant['location']),
                              ),
                            )
                            .toList(),
                  ))
              .toList(),
        ),
      ),
    );
  }
}

final data = [
  {
    "types": "ARABIAN",
    "restaurants": [
      {
        "name": "ZauZau Restaurants",
        "location": "Mascat, Oman",
        "kms": "1.0 kms"
      },
      {
        "name": "MauMau Restaurants",
        "location": "Calicut, Oman",
        "kms": "2.0 kms"
      },
      {
        "name": "ChaCha Restaurants",
        "location": "Charat, Oman",
        "kms": "2.5 kms"
      },
    ]
  },
  {
    "types": "COFFEE SHOP",
    "restaurants": [
      {
        "name": "ZauZau Restaurants",
        "location": "Mascat, Oman",
        "kms": "1.0 kms"
      },
    ]
  },
];

Advanced Full Solution

And here is a second solution providing exactly the same visual but managing specific Domain Entities Restaurant & RestaurantCategory.

It introduces concepts such as State Management and JSON Serialization.

Full source code:

import 'package:flutter/material.dart';
import 'package:flutter_hooks/flutter_hooks.dart';
import 'package:freezed_annotation/freezed_annotation.dart';
import 'package:hooks_riverpod/hooks_riverpod.dart';

part '66318447.tabs.freezed.dart';
part '66318447.tabs.g.dart';

void main() {
  runApp(
    ProviderScope(
      child: MaterialApp(
        debugShowCheckedModeBanner: false,
        title: 'Flutter Demo',
        home: HomePage(),
      ),
    ),
  );
}

class HomePage extends HookWidget {
  @override
  Widget build(BuildContext context) {
    final data = useProvider(dataProvider).state;
    return DefaultTabController(
      length: data.length,
      child: Scaffold(
        appBar: AppBar(
          title: Text('Restaurants'),
          bottom: TabBar(
            tabs: data.map((category) => Tab(text: category.name)).toList(),
          ),
        ),
        body: TabBarView(
          children: data
              .map((category) => ListView(
                    children: category.restaurants
                        .map(
                          (restaurant) => ListTile(
                            title: Text(restaurant.name),
                            subtitle: Text(
                                '${restaurant.location} (${restaurant.kms} away)'),
                          ),
                        )
                        .toList(),
                  ))
              .toList(),
        ),
      ),
    );
  }
}

final dataProvider = StateProvider<List<RestaurantCategory>>(
    (ref) => data.map((json) => RestaurantCategory.fromJson(json)).toList());

@freezed
abstract class Restaurant with _$Restaurant {
  const factory Restaurant(String name, String location, String kms) =
      _Restaurant;

  factory Restaurant.fromJson(Map<String, dynamic> json) =>
      _$RestaurantFromJson(json);
}

@freezed
abstract class RestaurantCategory with _$RestaurantCategory {
  const factory RestaurantCategory(String name, List<Restaurant> restaurants) =
      _RestaurantCategory;

  factory RestaurantCategory.fromJson(Map<String, dynamic> json) =>
      _$RestaurantCategoryFromJson(json);
}

final data = [
  {
    "name": "ARABIAN",
    "restaurants": [
      {
        "name": "ZauZau Restaurants",
        "location": "Mascat, Oman",
        "kms": "1.0 kms"
      },
      {
        "name": "MauMau Restaurants",
        "location": "Calicut, Oman",
        "kms": "2.0 kms"
      },
      {
        "name": "ChaCha Restaurants",
        "location": "Charat, Oman",
        "kms": "2.5 kms"
      },
    ]
  },
  {
    "name": "COFFEE SHOP",
    "restaurants": [
      {
        "name": "ZauZau Restaurants",
        "location": "Mascat, Oman",
        "kms": "1.0 kms"
      },
    ]
  },
];