That's hard to answer because the advantage is essentially infinitely large. Engines never lose or draw against unassisted humans. Any modern chess engine, if it plays 100 games against any human (even magnus), will have a record of 100 wins, 0 draws, and 0 losses. This is true both in standard chess and Chess960.

This is most definitely wrong. There will be some draws and even a few wins, though that's very rare indeed.