Using 2019 27 inch iMac as main computer, how do I add my 2018 MacBook Air as a second monitor?

You can't. Only certain iMacs can be used in Target Screen Mode.

The following is from this Apple document; Use your iMac as a display - Apple Support

Use your iMac as a display

You can view the desktop of your Mac on the display of some iMac models using Target Display Mode. In some cases, you can also use Target Display Mode to play the sound from your Mac (called the primary Mac) on the speakers of the external iMac. For example, if you have a MacBook Pro you could use an iMac as the display and for playing audio.

Note: Target Display Mode isn’t supported on iMac models with Retina display. Only iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), and iMac (Mid 2011 to Mid 2014) support Target Display Mode.

Neither one will work as a second monitor for the other one...

• 27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable to connect your iMac to a Mini DisplayPort or Thunderbolt or Thunderbolt 2 port on the other Mac.
• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display. It does not work with macOS Mojave or later.