They don't, they fly as cleared and report if they have to deviate, when they can.
This plane could be anywhere in the damned ocean.
The likelihood is still the Gulf of Siam(Thailand) although there was reporting they are also looking at the N end of the Straits of Malacca .
A pilot on TV tonight said lots of pilots turn the auto pilot on at 400 feet and don’t turn it back on until they’re almost on the runway.