What is the difference between the different font smoothing levels on macOS?
I think I've found the most reasonable explanation for how Font Smoothing works in macOS here.

Noting some of the good observations from that thread.

Sub-pixel rendering in Mojave (M) doesn't work the same way it does in High Sierra (HS). Specifically, in HS, gray base text most stays dark gray, with colored pixels get added around it when you enable sub-pixel rendering. In Mojave, much of the base text is changed from gray to other, lighter colors, thus reducing contrast. As much of Apple's menu text is gray rather than black, this effect reduces contrast throughout the OS.

In Mojave, font smoothing doesn't actually make the letters thicker (i.e., it doesn't darken more pixels). Rather, it makes the darkened pixels darker, thus giving the appearance of added thickness.

These observations should apply to macOS Catalina as well.