The terminology is rather confusing and inconsistent.
Physical on-hit effects don't usually apply lifesteal (i.e. Kindred's passive, Bloodrazor) and Blade of the Ruined King is an explicit exception.
"Modified" autoattacks (i.e. Sheen, Xin Zhao's Q, Trundle's Q, Nasus' Q, Draven's Q, etc) will generally have lifesteal applies based on the bonus damage.
However, Xin Zhao's Q is apparently an on-hit effect (since it procs after two autoattacks with Rageblade).
In addition, I don't believe Illaoi's W applies lifesteal based on their bonus damage either even though it is a "modified" autoattack.
At the moment, you usually need to know how each individual ability works.
Riot probably has to do a consistency pass for physical damage on-hit effects (for terminology if nothing else) to clear the situation up.
Note: I don't believe Modified Autoattacks are an official term or thing. They do reasonably describe a subset of physical damage on-hit effects, but there are exceptions and inconsistencies.