buildscript和allprojects的作用和区别是什么？

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    repositories {
        jcenter()
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
//        maven{ url 'http://maven.aliyun.com/nexus/content/groups/public/'}
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.3.1'
    }
}

allprojects {
    repositories {
        maven { url 'https://maven.google.com' }
        jcenter()
        mavenCentral()
        maven { url "https://dl.google.com/dl/android/maven2/"}
//        maven{ url 'http://maven.aliyun.com/nexus/content/groups/public/'}
    }
}