To enable the classic old legacy Search Box in Explorer on Windows 10, you can tweak the Registry or use Mach2. Do it if you do not like current default behavior.

Here’s how it works. Once you upgrade to the Windows 10 May 2019 Update (version 1903), you'll notice something different in the taskbar. You'll see a search box and a Cortana button next to it.

You can now customize the Search tool through its own dedicated settings page in the Windows 10 May 2019 Update.

When you double-click on the file, Windows 10 will display a UAC prompt asking if you would like to allow Registry Editor to make changes to your system. Click on the Yes button to continue.