Mode C Altitude Q - no Kalisman Correction? (Barometric Pressuredeviation correction?)

Being a student pilot, I'm eating up all kinds of info.
So I go through various instrument info, and then get
to the "Mode C Transponder" that sends altitude info.

Is that alt info built in?

If so, is there no correction for barometric pressure changes?

I don't know that it would be severe, but it seems like the
window could be +/- 100 feet from day to day that the controllers
get from the transponder.

Anyone provide some more info on this issue?

Thanks!

Gary