Fractional scaling is rendering directly to a framebuffer at the target monitors native resolution, and scaling your UI fractionally to look right on the target screen size.

Apples approach is more resource intensive, but probably makes UI layout implementation a lot simpler and more consistent + potentially looks better than rendering at "real" native res for a lot of content.

4k at 1x produces UI that's too small, 2x scaling is too large for a 27 inch monitor. 1.5x sizes everything like a 1440p display but you still get the higher resolution rendering at 4k.

Fractional scaling _is_ rendering at the 'proper' resolution of the display. It can be challenging to workaround some issues like how to deal with window sizes that scale to a fractional native buffer (i.e. a 501 logical pixel wide image becomes 751.5 physical pixels?). Apple decides 'no' to native fractional scaling, so does GNOME unless that's changed recently.

What I generally do on my Gnome systems running 4k screens is to use 125% or 150% scaling, and then set larger default fonts for the system. Fractional scaling helps keep old apps legible even if imperfect, but updated apps which respect system font size render great (Gtk+ widgets auto-scale to fit the text).

Unfortunately, this approach ain't gonna work with Wayland as well.