Build.cs

using System;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
using UnityEditor;
using UnityEngine;
using Application = UnityEngine.Application;
using BuildResult = UnityEditor.Build.Reporting.BuildResult;

public class Build : MonoBehaviour {
    static readonly string ProjectPath = Path.GetFullPath(Path.Combine(Application.dataPath, ".."));

    static readonly string apkPath = Path.Combine(ProjectPath, "Builds/" + Application.productName + ".apk");

    private static readonly string androidExportPath =
        Path.GetFullPath(Path.Combine(ProjectPath, "../../android/UnityExport"));

    private static readonly string iosExportPath =
        Path.GetFullPath(Path.Combine(ProjectPath, "../../ios/UnityExport"));

    [MenuItem("ReactNative/Export Android (Unity 2019.3.*) %&n", false, 1)]
    public static void DoBuildAndroidLibrary() {
        DoBuildAndroid(Path.Combine(apkPath, "unityLibrary"));

        Copy(Path.Combine(apkPath, "launcher/src/main/res"), Path.Combine(androidExportPath, "src/main/res"));
    }

    [MenuItem("ReactNative/Export Android legacy %&a", false, 2)]
    public static void DoBuildAndroidLegacy() {
        DoBuildAndroid(Path.Combine(apkPath, Application.productName));
    }

    public static void DoBuildAndroid(String buildPath) {
        if (Directory.Exists(apkPath)) {
            Directory.Delete(apkPath, true);
        }
        if (Directory.Exists(androidExportPath)) {
            Directory.Delete(androidExportPath, true);
        }

        EditorUserBuildSettings.androidBuildSystem = AndroidBuildSystem.Gradle;

        var options = BuildOptions.AcceptExternalModificationsToPlayer;
        var report = BuildPipeline.BuildPlayer(
            GetEnabledScenes(),
            apkPath,
            BuildTarget.Android,
            options
        );

        if (report.summary.result != BuildResult.Succeeded) {
            throw new Exception("Build failed");
        }

        Copy(buildPath, androidExportPath);

        // Modify build.gradle
        var build_file = Path.Combine(androidExportPath, "build.gradle");
        var build_text = File.ReadAllText(build_file);
        build_text = build_text.Replace("com.android.application", "com.android.library");
        build_text = build_text.Replace("bundle {", "splits {");
        build_text = build_text.Replace("enableSplit = false", "enable false");
        build_text = build_text.Replace("enableSplit = true", "enable true");
        build_text = build_text.Replace("implementation fileTree(dir: 'libs', include: ['*.jar'])", "implementation ':unity-classes'");
        build_text = Regex.Replace(build_text, @"\n.*applicationId '.+'.*\n", "\n");
        File.WriteAllText(build_file, build_text);

        // Modify AndroidManifest.xml
        var manifest_file = Path.Combine(androidExportPath, "src/main/AndroidManifest.xml");
        var manifest_text = File.ReadAllText(manifest_file);
        manifest_text = Regex.Replace(manifest_text, @"<application .*>", "<application>");
        Regex regex = new Regex(@"<activity.*>(\s|\S)+?</activity>", RegexOptions.Multiline);
        manifest_text = regex.Replace(manifest_text, "");
        File.WriteAllText(manifest_file, manifest_text);
    }

    [MenuItem("ReactNative/Export IOS (Unity 2019.3.*) %&i", false, 3)]
    public static void DoBuildIOS() {
        if (Directory.Exists(iosExportPath)) {
            Directory.Delete(iosExportPath, true);
        }

        EditorUserBuildSettings.iOSBuildConfigType = iOSBuildType.Release;

        var options = BuildOptions.AcceptExternalModificationsToPlayer;
        var report = BuildPipeline.BuildPlayer(
            GetEnabledScenes(),
            iosExportPath,
            BuildTarget.iOS,
            options
        );

        if (report.summary.result != BuildResult.Succeeded) {
            throw new Exception("Build failed");
        }
    }

    static void Copy(string source, string destinationPath) {
        if (Directory.Exists(destinationPath))
            Directory.Delete(destinationPath, true);

        Directory.CreateDirectory(destinationPath);

        foreach (string dirPath in Directory.GetDirectories(source, "*",
            SearchOption.AllDirectories))
            Directory.CreateDirectory(dirPath.Replace(source, destinationPath));

        foreach (string newPath in Directory.GetFiles(source, "*.*",
            SearchOption.AllDirectories))
            File.Copy(newPath, newPath.Replace(source, destinationPath), true);
    }

    static string[] GetEnabledScenes() {
        var scenes = EditorBuildSettings.scenes
            .Where(s => s.enabled)
            .Select(s => s.path)
            .ToArray();

        return scenes;
    }
}