The EPSG code for 3857 has this in its "Remarks" section (https://epsg.org/crs_3857/WGS-84-Pseudo-Mercator.html) :
Not a recognised geodetic system. Uses spherical development of ellipsoidal coordinates. Relative to WGS 84 / World Mercator (CRS code 3395) gives errors of 0.7 percent in scale and differences in northing of up to 43km in the map (21km on the ground).
Does "Spherical development of ellipsoidal coordinates" mean that it takes WGS84 coordinates, which are geodetic lat-long on the ellipsoid as defined in EPSG:4326, and ignores the ellipsoid, pretends they are lat-long on a perfect sphere, and then does the Mercator transform?
And this is in contrast to EPSG:3395, which treats EPSG:4326 coordinates correctly as ellipsoidal coordinates, and does the Mercator transform with them?
I can't find any clear explanation of what that phrase means, and I want to make sure I'm interpreting it correctly.