Q:

What is the default effective resolution of the Macbook Air 2020

The Internet says that the Macbook Air's resolution is 2560x1600, and that its supported scaled resolutions are: 

1680 by 1050
1440 by 900
1024 by 640

But which of those supported scaled resolutions is the effective resolution of the Macbook by default? Or maybe it's not even listed. If you use the non-default scaled resolutions, they don't look quite right to my eyes, and so I want to know how small or large everything is. Most software doesn't factor in hardware pixels when choosing what size to display.
For example, the Macbook Pro 16-inch has a native 3072x1920 resolution, and its supported scaled resolutions are:

2048 x 1280
1792 x 1120
1344 x 840
1152 x 720

#2 is the effective resolution when running in default retina mode, giving the Macbook Pro a PPI of 132.08

A:

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.

