1) The Yamnaya were a combination of majority eastern europeans and minority west eurasian caucuses. The indo iranians (a linguistic category) are a branch (child of) the indo europeans, that are a secondary admixture as the technology and culture of the yamaya moved eastward by expansion and adoption.
2) “Later steppe populations tied to the Indo-Iranian expansion are typically modeled as a Middle–Late Bronze Age steppe cluster that already includes dilution by substantial extra ancestry from European farmers.”
3) ““Later Indo-Iranians” (after movement south) are steppe ancestry diluted by admixture with local populations. Once you move from “steppe source” to “Indo-Iranian–speaking populations in Iran / Afghanistan / Pakistan / India”, you’re no longer comparing two discrete groups; you’re comparing a cline created by repeated mixing.”

Long time friend Simon Strong posts one of his insights with a video on the West Eurasian Holocene