Re-Install MacOS Sierra I have 2017 MacBook Air, which has been upgrade to Mojave. Sierra was installed at the time of purchase.

Now, I need to refresh the Mac as brand new device. Therefore, I am trying to erase the hard disk. Below is a image of the Disk Utility. Which is the one to be erased? Are they 2 hard disks or 1?

I was able to erase the AppleAPF, but was unable to do anything with the Apple SSD, which appears to be unformatted.



After erasing the AppleAPFS, I started the installation process, but this took a very long time (lots of hours) with nothing moving. I do not know if this because my internet connection is very week or due to something else.
You have to select AppleAPFS on the top, click Erase but if you try to run recovery option, it will install Mojave again. So to have Sierra, I would suggest the following steps:


First, connect your Macbook with an ethernet cable as it takes an hour to get full installation (basically depends upon the internet speed also). 
After this, turn off your computer and Reinstall from macOS Recovery mode with a combination of Shift-Option-⌘-R (this allows to install the macOS that came with your Mac, or the closest version still available.)


Basically, as per Apple Guidance:

Command (⌘)-R
Install the latest macOS that was installed on your Mac.

Option-⌘-R
Upgrade to the latest macOS that is compatible with your Mac.

Shift-Option-⌘-R
Install the macOS that came with your Mac, or the closest version still available.

Let me know how it goes.