How to Fix the Question Mark When I Factory Reset My MacBook?

Hi there. The question mark indicates that there is no OS installation that your internal HDD/SDD can recognize. I recommend that you enter Internet Recovery mode to download a macOS instead.

Let me show you how to enter Internet Recovery mode on your MacBook Air 2015:

1. Completely shut down your Mac.

2. Press the power button or Touch ID button and immediately hold down Command + Option + R or Command + Shift + Option + R key combination.

3. Hold the keys until you see a spinning globe appears on the screen with the message "Starting Internet Recovery. This may take a while."

4. Select a Wi-Fi and enter your Wi-Fi password if asked when a Wi-Fi menu appears. Starting your Mac in Internet Recovery Mode requires a network connection to load tools from Apple's servers.

5. Wait until the Mac boots to Internet Recovery. It may take minutes or even hours depending on your Internet speed, so make sure your Mac is fully charged. 

You will see the same macOS Utilities window as in standard macOS Recovery with all tools available where you can reinstall macOS.

Step 1. Click the Reinstall macOS option and click Continue.

Step 2. Select a destination to install the new copy of macOS and select the startup disk you formatted earlier.

Step 3. Click Continue to begin the reinstallation. You will see a window with a progress bar and time remaining for installing OS.

It will take a while and do not let your computer shut down or sleep. After the macOS installation process completes, your Mac restarts automatically to a setup assistant and then asks you to create an admin account.

Over here, you have completed your factory setting and macOS reinstallation.

Also read: 

How to Completely Reset a MacBook Pro/Air to Factory Settings