Android Top-Level Build File, Project Wide properties

Gradle settings file located in the root of the project directory  tells Gradle which modules it should include when building your app

for e.g. include ':app', ':appnotifications', ':appmessaging', ':applib'
The top-level build.gradle file, located in the root project directory, defines build configurations that apply to all modules in your project.

By default, the top-level build file uses the buildscript block to define the Gradle repositories and dependencies that are common to all modules in the project.
Below is from Google documentation

/**
 * The buildscript block is where you configure the repositories and
 * dependencies for Gradle itself—meaning, you should not include dependencies
 * for your modules here. For example, this block includes the Android plugin for
 * Gradle as a dependency because it provides the additional instructions Gradle
 * needs to build Android app modules.
 */

buildscript {

    /**
     * The repositories block configures the repositories Gradle uses to
     * search or download the dependencies. Gradle pre-configures support for remote
     * repositories such as JCenter, Maven Central, and Ivy. You can also use local
     * repositories or define your own remote repositories. The code below defines
     * JCenter as the repository Gradle should use to look for its dependencies.
     *
     * New projects created using Android Studio 3.0 and higher also include
     * Google's Maven repository.
     */

    repositories {
        google()
        jcenter()
    }

    /**
     * The dependencies block configures the dependencies Gradle needs to use
     * to build your project. The following line adds Android plugin for Gradle
     * version 3.1.0 as a classpath dependency.
     */

    dependencies {
        classpath 'com.android.tools.build:gradle:3.1.0'
    }
}

/**
 * The allprojects block is where you configure the repositories and
 * dependencies used by all modules in your project, such as third-party plugins
 * or libraries. However, you should configure module-specific dependencies in
 * each module-level build.gradle file. For new projects, Android Studio
 * includes JCenter and Google's Maven repository by default, but it does not
 * configure any dependencies (unless you select a template that requires some).
 */

allprojects {
   repositories {
       google()
       jcenter()
   }
}

Configure project-wide properties

buildscript {...}

allprojects {...}

// This block encapsulates custom properties and makes them available to all
// modules in the project.
ext {
    // The following are only a few examples of the types of properties you can define.
    compileSdkVersion = 26
    // You can also create properties to specify versions for dependencies.
    // Having consistent versions between modules can avoid conflicts with behavior.
    supportLibVersion = "27.1.1"
    ...
}

to use these variables from inner modules, below can be done

android {
  // Use the following syntax to access properties you defined at the project level:
  // rootProject.ext.property_name
  compileSdkVersion rootProject.ext.compileSdkVersion
  ...
}
...
dependencies {
    compile "com.android.support:appcompat-v7:${rootProject.ext.supportLibVersion}"
    ...
}

This is a good note from Google documentation

Note: Although Gradle allows you to define project-wide properties at the module level, you should avoid doing so because it causes the modules that share those properties to be coupled. Module coupling makes it more difficult to later export a module as a stand-alone project and effectively prevents Gradle from utilizing parallel project execution to speed up multi-module builds.

references:

https://developer.android.com/studio/build/#settings-file