OT. Ot is used to help people learn compensatories strategies for performing their activities of daily living including bathing, grooming, dressing, getting in and out of the bathroom, showering etc, they also focus on trying to strengthen the upper extremities deal with issues of pain and paralysis of the upper extremities. They are very helpful. It has nothing to do with an "occupation".
Plasticity. Though a certain part of the brain dies in a stroke (those neurons/ brain cells are lost forever) the surviving neurons can often form new connections to at least partially take over for the dead neurons. Therapy exercises helps to force the surviving neurons to form these new connections.