from top of my head, I'm not expert in this field.
Just like europeans are 3 way mixture between farmer, IE steppe and local HG. South Asia also had 3 way mixture.
Europeans:
1: Local HG
2: farmers from middle east/anatolia
3: IE invasions from western steppe
south asians:
1: Local HG/AASI like people
2: neolithic farmers
3: Steppe IE invasions
Now keep in mind I'm not saying these people migrated from south india etc IVC is indigenous. Every population in the world is mixed if one go back long enough then we all end up in Africa and same. Since there is no evidence of IVC like civilisation existing in south india or for that matter outside limits of indus valley, its clear they were not recent migrants/invaders. Unlike IE languages which we know where they came from.
Indigenous population of IVC was as one should have expected two way mix and their admixture composition was similar to middle caste dravidians with no steppe admixture. This doesn't mean dravidians migrated to IVC because this mix is pretty much standard in south asia. And even north Pakistan have much less steppe ancestry then european populations.
https://www.biorxiv.org/content/early/2018/03/31/292581