Short answers, pulled from the story.
Southern Buddhism corresponds to Theravada, Eastern Buddhism to Mahayana as practiced in China, Japan, and Korea, and Northern Buddhism to Tibetan and Mongolian Vajrayana traditions. The terms are geographical labels sometimes used to map the three principal Buddhist branches across Asia.
The Pali Canon is the body of sacred scriptures written in the Pali language that serves as the textual authority for Southern Buddhism. Southern Buddhist communities in countries such as Sri Lanka, Cambodia, Myanmar, Thailand, and Laos are united primarily by their strict adherence to the Pali Canon alongside a strong monastic tradition.
The four main schools of Tibetan Buddhism are Kagyu, Sakya, Nyingma, and Geluk. Each school is associated with particular doctrinal positions or specialized practices, but they are organized around lineages of teachers rather than rigid sectarian boundaries.
Theravada began to dominate Southeast Asia only after Buddhism declined in India. Before the 13th century, Mahayana was well established across the region. Once Buddhism faded in India, Sri Lanka replaced India as the primary source of new texts and teachers for Southeast Asian communities.
Tibetan Buddhism draws its roots from the Pala dynasty, which ruled Bengal and Bihar from the 8th to the 12th centuries CE. That dynasty's monastic universities produced the Buddhist scholarship, Mahayana philosophy, and tantric traditions that Tibetan scholars translated into their language and built upon.
Vietnam sits in the boundary zone between the Northern and Southern Buddhist worlds, and its Buddhist practice shows both strong Mahayana and Theravada influence. That dual inheritance makes Vietnam unusual among Asian Buddhist countries, most of which align more clearly with one tradition.