GPS
...
GPS
Actually GPS coupled with an onboard computer that steers the tractor.
...and in some cases the additional aid of a drone running ahead of the tractor to scope out obstacles (rocks, washouts, etc).
BIL has one that has more electronics than I can recall seeing on front-line Navy aircraft back in The Day.....I've teased him about sending the tractor out to plow while he's back in the kitchen drinkin' coffee.
And the so-and-so just grinned at me.