explain how the animal came to be in possession of the farm

The animals staged a rebellion in which they took over the farm.

"On Midsummer’s Eve in June, Mr. Jones gets so drunk that he passes out and neglects to feed the animals. Having gone unfed for hours, the animals break into the store-shed and eat. Mr. Jones and the farmhands rush in and begin whipping the animals indiscriminately, and the animals respond by attacking them in unison. The men are frightened and forced to flee the farm.

After a disbelieving calm, the animals barge into the harness-room and drown or burn all the implements of their former bondage. Snowball makes sure to burn the ribbons, which he calls tantamount to clothing, and states, “All animals should go naked” (40). The animals then help themselves to double servings of food and sleep better than they ever have. When they awake the next morning, they survey the farm with new eyes, absorbing the fact that it is now their own. Finally, they tour the farmhouse, seeing in disbelief the “unbelievable luxury” in which the Joneses had lived. Then the animals agree to leave the farmhouse intact as a museum. They confiscate a few hams for burial and leave."