This video is worth watching. It explains clearly how people migrated to India from Africa and then from other parts step by step, instead of Aryans invading suddenly.
It also shows how North India and South India became different in DNA mixing over time because of these slow migrations in different shades.
One important point is about caste:
From around 100 BCE, people from different castes stopped intermarrying (marrying between different groups), and this eventually stopped DNA mixing.
DNA of women did not mix as much. This means men from one group migrated here without women and often married women here.
