Who adds the "start transaction" and "commit" to the intended SQL statement in "autocommit" mode?

I’ve searched in vain for an account of how "autocommit" mode actually works. (I tried the built-in search feature
withinthe PG docs. And I tried Google.) 

It seems clear enough that turning "autocommit" mode "on" or "off" is done by using a client-env-specific command like
"\set"is psql, or "SET" in ECPG (Embedded SQL in C) or "self.session.set_session(autocommit=...)" in Python. And that
themode is a property of the current session. 

But it's not clear who actually implements the opening "start transaction" and the closing "commit" around every
submittedSQL statement when autocommit is "on". 

Is this done in client-side code (maybe implying three round trips per intended SQL statement)? Or is it done
server-side?