How to make Android apps which support both 32-bit and 64-bit architecture?

According to an official email sent by the Google Play Team, the action required is:

If you haven’t yet, we encourage you to begin work for the 64-bit
requirement as soon as possible. Many apps are written entirely in
non-native code (e.g. the Java programming language or Kotlin) and
will not need code changes.

Please note that we are not making changes to our policy on 32-bit
support. Google Play will continue to deliver apps with 32-bit native
code to 32-bit devices. The requirement means that those apps will
need to have a 64-bit version as well.

To help you make the transition, we’ve prepared documentation on how
to check whether your app already supports 64-bit and how to become
64-bit compliant.

We’re also providing a high-level timeline below.

So, the linked documentation explains:

If your app uses only code written in the Java programming language or
Kotlin, including any libraries or SDKs, your app is already ready for
64-bit devices. If your app uses any native code, or you are unsure if
it does, you will need to assess your app and take action.

[…]

The simplest way to check for 64-bit libraries is to inspect the
structure of your APK file. When built, the APK will be packaged with
any native libraries needed by the app. Native libraries are stored in
various folders based on the ABI. It is not required to support every
64-bit architecture, but for each native 32-bit architecture you
support you must include the corresponding 64-bit architecture.

For the ARM architecture, the 32-bit libraries are located in
armeabi-v7a. The 64-bit equivalent is arm64-v8a.

For the x86 architecture, look for x86 for 32-bit and x86_64 for
64-bit.

The first thing to do is ensure that you have native libraries in both
of these folders.[…]

And, to build 64-bit libraries, you basically need to follow the instructions below:

Most Android Studio projects use Gradle as the underlying build
system, so this section applies to both cases. Enabling builds for
your native code is as simple as adding the arm64-v8a and/or x86_64,
depending on the architecture(s) you wish to support, to the
ndk.abiFilters setting in your app’s ‘build.gradle’ file:

// Your app's build.gradle
apply plugin: 'com.android.app'

android {
   compileSdkVersion 27
   defaultConfig {
       appId "com.google.example.64bit"
       minSdkVersion 15
       targetSdkVersion 28
       versionCode 1
       versionName "1.0"
       ndk.abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
// ...

Finally, a quick note:

The 64-bit version of your app should offer the same quality and
feature set as the 32-bit version.

By the way, this official video talks a little bit about it.