This is what I was able to research as the most probable.
In terms of the research, what I have found the most accurate was of a multi-faceted wave of Indian people groups, with the last being a top-down cultural diffusion of nomadic pastoralists of Central Asia. The first wave to reach the Indian subcontinent was circa 50,000 years ago, a key branch in the great human migrations, these people had a “physiological profile most similar to modern Australian aborigines and New Guineans, are then taken to be the most representative of this ancient population, which has been labeled by geneticists as Ancient Ancestral South Indians (AASI),”(Pillalamarri 2019). These people were primarily hunter-gatherers and had no use of both agriculture and metallurgy. The second wave was the one that would see the rise of agriculture in the Indian subcontinent, “Some farmers from western Iran, one of the homelands of agriculture, migrated east to the Indus Valley in modern Pakistan by 9,000-7,000 years ago, where they remained without further expansion for a few thousand of years”(Pillalamarri 2019). These wave of Iranian agriculturalists contribute most of the genome to South Asian populations and were responsible for the founding of the Indus Valley Civilization. This would be followed by a wave of Austro-Asiatic migrants “the original AASI-like inhabitants of Southeast Asia...[]...these rice farmers reached India around 4,000 years ago,” (Pillalamarri 2019). This would be followed by Indo-European nomadic pastoralists “group of Aryans spread southeast through the Hindu Kush range of Afghanistan into South Asia; these are the Indo-Aryans most prominently known in history”(Pillalamarri 2019). Arriving around 1500 BC, these Aryans would contrary to common belief, simply diffuse and eventually mix with the larger established agricultural populations. The reason for Indo-Aryan dominance may be as innocuous as factors like “elite dominance, it is not uncommon for smaller groups of warriors and rulers to spread their culture and language to larger settled groups, especially those bereft of leadership”. There is simply no evidence for the Aryan invasion theory of military force present in South Asian anthropology. A lack of both military conquest in the archaeological record “subsequent examinations of the skeletons by Kenneth Kennedy in 1994 showed that the marks on the skulls were caused by erosion, and not by violence (Bryant 159) and the fact that the Indus Valley civilization collapsed centuries earlier thoroughly discredits the Aryan-Dasyu Invasion Theory. Furthermore, “the dichotomy of Aryans/Dasyus apparently is a highly simplistic presentation of the fact. More and more recent investigations are now rejecting the earlier and more popular version of invading Aryan hordes”. This is further compounded by the lack of mention of caste in the Rigveda, one of the earliest documents relating to Hinduism composed in South Asia (Roy 443). It is these features that thoroughly discredits any invasion theory. The Aryans were an honorific for a diverse group of cultures and nomads that gradually integrated into Indian society over a period of years through a system of top-down cultural diffusion. The caste system, with its end in endogamy and sadly the proliferation of untouchability would become “fairly well-established that the system hardened particularly during the Gupta (320-550 CE) period” (Pillalamarri 2019). A minority opinion is present that argues the caste system became prominent only in and as a result of the British colonial era (Ghuman 2011). However, it is more likely in the Gupta Era that indigenous Indian tribes would join the society at the bottom of the caste system, with the growing strength of Brahminism that would lead to bigger and bigger social divides.
I gave the citations but if you ask for the sources I will oblige.