Lawn tractors are usually two wheel drive with an open rear end so only one wheel is driven and it is always the wheel that can spin the easiest.

Unless the specifications state that there is a differential lock you can assume two wheel drive.

Some garden tractors have a locking differential so both rear wheels can be driven when required by engaging the diff lock.

Top tier garden tractors are available with AWD and 4WD.

2 wheel drive axles (car, truck, tractor) with a standard type of differential DIVIDE the power equally to both wheels (50% to each wheel) so long as tractive conditions are roughly the same for each wheel.
However, when one wheel has less traction, that wheel can lose its grip. As soon as the wheel with less traction begins to slip, all power through the differential gears shifts to that side and you are stuck.
LOCKING differentials feature some extra components that can lock the 2 wheel axles together and prevent the differential gears from sending all power to 1 wheel.
The locking feature may be automatic, or it may be the type you have to engage manually as conditions change.