Short answers, pulled from the story.
Sentient beings conventionally refer to the mass of living things subject to illusion, suffering, and rebirth known as sañsāra. This class broadly encompasses all beings possessing consciousness, including Buddhas and Bodhisattvas themselves.
Early Buddhist sources classify sentient beings into five categories including divinities, humans, animals, tormented spirits, and denizens of hell. Sometimes the classification adds another category called asuras between divinities and humans.
Mahayana Buddhism teaches that sentient beings also contain Buddha-nature, the intrinsic potential to transcend the conditions of sañsāra and attain enlightenment. Thus, it is to sentient beings that the bodhisattva vow of compassion is pledged.
Zhanran lived from 711 to 782 and defended the idea that insentient things possess Buddha nature while leading the Tiantai school. Japanese figures like Kūkai and Dōgen further developed these perspectives on non-living matter.
Sentient beings are composed of matter, sensation, perception, mental formations, and consciousness. These five components form the philosophical composition of any entity considered alive or conscious within the tradition.