Plugins section in Android main project build.gradle and missing modules

1. I get the following error:

BUILD FAILED in 56s ERROR: Plugin [id: 'de.undercouch.download'] was not found in any of the following sources:

- Gradle Core Plugins (plugin is not in 'org.gradle' namespace)
- Plugin Repositories (plugin dependency must include a version number for this source)

I want to know about this plugins section.

I am looking in the following guide:

Plugins in Android Gradle

But it is in bits and pieces and I want to see complete example of build.gradle file.

2. How to integrate the following in "/nbproject":

/@react-native-community/cli-platform-android/native_modules.gradle

all I can see is the following folder in the path:

/@react-native-community/cli

3. Missing react-native-unimodules in /nbproject. At the end of this post is my settings.gradle: So what command I should use instead of:

npm install

//apply from: "../../node_modules/react-native-unimodules/gradle.groovy"
apply from: "/Users/lion/Documents/Project/ReactNative/Libraries/rn_mac_kitchensync/node_modules/react-native-unimodules/gradle.groovy"
include ':react-native-screens'
project(':react-native-screens').projectDir = new File(rootProject.projectDir, '../node_modules/react-native-screens/android')
include ':react-native-reanimated'
project(':react-native-reanimated').projectDir = new File(rootProject.projectDir, '../node_modules/react-native-reanimated/android')
include ':react-native-gesture-handler'
project(':react-native-gesture-handler').projectDir = new File(rootProject.projectDir, '../node_modules/react-native-gesture-handler/android')
//==
include ':react-native-vector-icons'
project(':react-native-vector-icons').projectDir = new File(rootProject.projectDir, '../node_modules/react-native-vector-icons/android')
//
include ':ReactAndroid'
//
project(':ReactAndroid').projectDir = new File(
        rootProject.projectDir, '../node_modules/react-native/ReactAndroid')
//
includeUnimodulesProjects()
//includeUnimodules([
//        modulesPaths: ['../../node_modules']
//])

rootProject.name = 'NativebaseKitchenSink'

include ':app'