Factory reset of a MacBook Air 2011

In order to erase a Mac properly, all data will need to be erased. However, as you said, since your Mac is older and not receiving updates anymore, it may be safer to create a bootable macOS installer to use to erase your Mac and reinstall macOS rather than relying on internet recovery.

To begin, refer to this Apple Support article, Create a bootable installer for macOS - Apple Support, and follow the steps to create a High Sierra bootable installer. You'll need an external storage device, that can be erased, for this.

Next, you can follow these steps to erase your Mac:

1. Turn on your Mac, with the installer connected, then immediately press and hold the Option (Alt) key.
2. Release the Option key when you see a dark screen showing your bootable volumes.
3. Select the volume containing the bootable installer. Then click the onscreen arrow or press Return. 
4. From the utilities window, select Disk Utility and click Continue.
5. Select Macintosh HD in the sidebar of Disk Utility.
6. Click the Erase button in the toolbar, then enter the requested details:
   1. Name: Macintosh HD
   2. Format: APFS or Mac OS Extended (Journaled), as recommended by Disk Utility
7. Click Erase.
8. When done, quit Disk Utility to return to the utilities window.
9. If you want your Mac to start up again from the volume you just you erased, select Reinstall macOS in the utilities window, then click Continue and follow the onscreen instructions.

-Jack

Hi there!

Just to confirm, you'd like to erase your MacBook Air, but not delete the system files, only your personal user files?

-Jack