Question: how to connect a second monitor to my macbook air?
Answer: Connect a VGA display or projector: Use a USB-C VGA Multiport Adapter to connect the display to a Thunderbolt 3 (USB-C) port on your MacBook Air.
Connect an HDMI display or HDTV: Use a USB-C Digital AV Multiport Adapter to connect the HDMI display or HDTV to a Thunderbolt 3 (USB-C) port on your MacBook Air.