Short answers, pulled from the story.
Buddhism was formally introduced to Tibet in the year 618 when King Songtsän Gampo ordered Sanskrit Buddhist scriptures to be translated into Tibetan. This event marked the beginning of Buddhism's presence during the Tibetan Empire.
Indian scholars Padmasambhāva and Śāntarakshita arrived at the court of King Trisong Detsen around the late eighth century to guide the process. They are considered the founders of the Nyingma tradition, known as The Ancient Ones.
The Indian Madhyamaka Middle Way philosophy dominates thought within this tradition and is also called Śūnyavāda or emptiness doctrine. In Madhyamaka the true nature of reality refers to Śūnyatā which means all phenomena are empty of inherent existence.
Large teaching monasteries housed thousands of monks such as Sera with over 6000 monks in the first half of the twentieth century. Drepung monastery held over 7000 monks during that same period.
The year 1959 saw the fourteenth Dalai Lama flee country along a great number of clergy and citizenry who settled in India and other neighboring countries. Events like the Cultural Revolution spanning from 1966 to 1976 made religion a main political target for the Chinese Communist Party.