The ancient Indian contacts with South-East Asia were mainly due to trade, migration and cultural diffusion. They began around the 4th century BCE and continued until the 15th century CE. They involved various regions of India, especially Bengal, Gujarat, Tamil Nadu and Andhra Pradesh, and various countries of South-East Asia, especially Indonesia, Cambodia, Thailand, Malaysia and Vietnam123.

The causes of these contacts were:

The demand for spices, gems, gold, ivory and other exotic products from South-East Asia by the Mediterranean world and China. This stimulated Indian merchants to explore new markets and routes in South-East Asia14.
The political instability and social unrest in India due to foreign invasions, dynastic conflicts and religious persecutions. This prompted some Indian groups to seek refuge or opportunities in South-East Asia12.
The missionary zeal of Indian religions, especially Buddhism and Hinduism. This motivated some Indian monks, teachers and pilgrims to spread their faiths and teachings in South-East Asia13.
The chronology of these contacts were:

The earliest phase (4th century BCE - 3rd century CE) was marked by sporadic trade contacts between India and Funan (Cambodia-Vietnam), Srivijaya (Sumatra) and Java. Some Indian traders settled in these regions as colonists or intermediaries19.
The second phase (4th century CE - 7th century CE) was marked by increased trade contacts between India and Kambuja (Cambodia), Champa (Vietnam), Dvaravati (Thailand), Malayu (Malay Peninsula) and Bali. Some Indian rulers established political alliances or suzerainty over these regions as patrons or overlords19.
The third phase (8th century CE - 15th century CE) was marked by intensified cultural contacts between India and Angkor (Cambodia), Sailendra (Java-Sumatra), Mataram (Java), Majapahit (Java-Bali) and Pagan (Burma). Some Indian culture bearers influenced these regions as artists, architects, poets or teachers.
The original home of the migrants were:

The early migrants were mostly merchants from Bengal, Gujarat, Tamil Nadu and Andhra Pradesh who settled along the coastal areas of South-East Asia as traders or colonists19.
The later migrants were mostly Brahmins, Kshatriyas, Buddhists and Jains from various parts of India who moved to the interior areas of South-East Asia as priests, rulers, monks or teachers19.
The impact of these contacts on the art of Kambuja and Java were:

The art of Kambuja was influenced by both Hinduism and Buddhism from India. It developed a distinctive style of temple architecture that combined pyramidal towers, carved stone walls, moats and causeways. Some examples are Angkor Wat, Bayon and Banteay Srei. It also developed a rich tradition of sculpture that depicted Hindu gods, Buddhist bodhisattvas, kings and apsaras. Some examples are Vishnu reclining on Ananta Sesha at Angkor Wat, Avalokiteshvara with multiple arms at Bayon and Devi at Banteay Srei.
The art of Java was also influenced by both Hinduism and Buddhism from India. It developed a unique style of temple architecture that combined stupa-like domes, terraced platforms, niches and reliefs. Some examples are Borobudur, Prambanan and Mendut. It also developed a splendid tradition of sculpture that depicted Hindu epics, Buddhist stories, animals and demons. Some examples are the Ramayana panels at Prambanan, the Jataka tales at Borobudur and the Kala heads at Mendut.

- ! Answer from above.