Extending screen from MacBook Air M1 using 2019 iMac

At this time there is no feature for that in macOS.

Apple has previewed a new AirPlay to Mac feature in macOS Monterey so you might keep an eye out for that.

https://www.apple.com/macos/monterey-preview/features/

Target Display mode is dead, please review Target Display Mode (Target Display Mode specifically where it says:

To use any of these iMac models as an external display:

• The iMac used as a display must have macOS High Sierra or earlier installed.
• The other Mac that you're connecting it to must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed.
• The cable connecting the two Mac computers must be a Mini DisplayPort  cable.