To compile a 64-Bit Android App in Unity you will need an Android NDK package. Unity does not support the latest NDK version. Instead, it requires the specific version, namely, r16b.
Thus, you need to go to https://developer.android.com/ndk/downloads/older_releases.html and download the android-ndk-r16b-windows-x86_64.zip file. After you downloaded the file, extract its content to a destination of your choice.
Next, you need to adjust Build Settings. In the Unity Editor, click on File -> Build Settings…
In the Build Settings window, click on the button Player Settings.
In the Inspector, expand the tab Other Settings on the panel Player Settings.
Set Scripting Backend to IL2CPP. This will allow you to select the ARM64 architecture in the Target Architectures section.
Now, you can build a 64-Bit Android package of your application. During the build process, Unity will ask you for the path to the Android NDK. You will have to navigate to the folder you extracted the Android NDK package and select it.