What is the default effective resolution of the Macbook Air 2020
By default, Mac Retina displays are always at scaled to 2x the 'actual' resolution. So a 2560 x 1600 display will be a "Hi-res" version of 1280 x 800.

I suspect that Apple's list of "supported scaled resolutions" does not include the default 2x Retina setting. 
This is backed up by the wording used on my 15" MBP:


  2880 by 1800 pixels (Retina); scaled resolutions: 1920 by 1200, 1680
  by 1050, 1280 by 800, and 1024 by 640 pixels


The Scaling options in System Preferences offer 5 options: four of them being the scaled sizes and the middle button being the default. (I'm hovering the mouse over the 2nd from left in the the image.)



The 16" MBP's default resolution is 1536 x 960 "Hi-res".

You are right that "most software doesn't factor in hardware pixels when choosing what size to display" - that's the job of the OS. MacOS's use of 2x scaling is designed to be the clearest.

If the non-default scaled resolutions don't look right to your eyes (understandably so), then just use the default.