Macbook Air Full Factory Reset?
There are so many terms used in the question that indicate a misunderstanding of what a &quot;full factory reset&quot; is.
First off, your MacBook Air (or any computer and not limited to Apple) is not a phone;  there's no &quot;factory reset.&quot;  In fact, &quot;full factory reset&quot; is not correct either.  You can &quot;reset to factory defaults&quot; which is a term used for wiping the custom configurations of your device so that it will behave like it was just removed from the box.  It doesn't, however, return it to the OS/firmware that it originally shipped with.  If you applied any firmware updates, you can't roll those back - and it wouldn't be prudent to do so.

I want it to be like it just left the factory, at least in the software side. Like SMC and NVRAM reset etc.

The SMC and NVRAM have absolutely nothing to do with software.  The SMC handles power, battery charging, fan control, etc.  There's nothing in the SMC that has anything to do with how you personally interact with your computer; there's no identifiable information.  The NVRAM sets your pre-boot environment variables and your Find My Mac token.  There's nothing in NVRAM that's personally identifiable and necessary of resetting.
I wrote an a short piece on what Resetting the SMC and NVRAM actually does. (it's part of a broader answer).  It's not necessary to reset either of these.

Or deleting the disk resets the things that I mentioned?

No. I assume you mean erasing your disk.  Your disk contents has nothing to do with the SMC.  As far as your NVRAM goes, pretty much the only entries in NVRAM related to your disk is where the the boot and recovery partitions are located and this info rarely changes from computer to computer.  You could reset it, but it will likely go back to what it was before.

After deleting the disk and before installing the High Sierra (I will downgrade from Mojave to High Sierra) or after installing the High Sierra?

Why downgrade?  Most users would want to know that the Mac runs the latest and greatest operating system, not one from three years ago (2017).  However, if you want to go this route, all you have to do is boot using the keyboard combo ⇧ Shift⌘ Command⌥ OptionR.  This will install the version of macOS that came with your Mac or the closest version still available (this one reason is why &quot;full factory reset&quot; is not applicable to computers).
See:  How to reinstall macOS from macOS Recovery
TL;DR

And are there any other things that can be reset like SMC and NVRAM? If there are, please mention them.

As mentioned, SMC and NVRAM reset are useless - doing this will accomplish nothing.  It won't hurt anything if you do so, if you feel better doing it, have at it.  I just want to make you aware that these resets, that are way too often cited, aren't very relevant.
What you should do is the following:

Ensure you've backed up your data

Sign out of iTunes/Music (de-authorize your computer), iCloud, Messaging, etc.

Remove any accounts you may have added (like Mail, Exchange, Social networking, etc.)

Unpair any Bluetooth devices you may have connected

Remove any firmware and File Vault encryption passwords you may have enabled

Wipe your drive and reinstall macOS

⌘ CommandR - Install the latest macOS version installed with your Mac
⇧ Shift⌘ Command⌥ OptionR - Install the version that came with or closest to when your Mac was shipped from the factory
Before proceeding with the installation of  macOS, use Disk Utility from the Recovery Boot Menu to wipe your drive


Turn off your computer once you get to the &quot;Welcome&quot; screen.  When the new user gets it and turns it on for the first time, they will get the &quot;new computer experience&quot; like it came from the factory.


The key is to un-link your Mac from you both physically and online.  Removing firmware and File Vault passwords will ensure the new user can use the Mac without going back to you for them.  Wiping the drive ensures your data is gone and not just &quot;hidden&quot; in a different user account.  Removing online links (i.e. iCloud) ensures the new user can link it to their account and keeps you and your data out of the mix.