Short answers, pulled from the story.
Western Steppe Herders were a distinct ancestral population whose genetic component was first identified in Chalcolithic steppe individuals from around the start of the 5th millennium BC. They descended from a merger between Eastern Hunter-Gatherers and Caucasus Hunter-Gatherers, with the mixing zone identified between the middle Volga and the North Caucasus foothills.
Yamnaya ancestry is one of several names for Western Steppe Herder ancestry. The Yamnaya culture expanded across Eurasia around 3,000 BC, carrying high levels of WSH ancestry, and is linked by most linguists, archaeologists, and geneticists to the dispersal of Indo-European languages.
Based on osteological evidence, the average Yamnaya male stood 175.5 cm tall and had long-headed crania. A 2015 study found the Yamnaya had the highest ever calculated genetic selection for stature of any ancient population tested.
WSH ancestry peaks in Northern Europe, particularly among Irish, Norwegian, Swedish, Belarusian, Lithuanian, Scottish, and Icelandic populations. In South Asia, it is highest in the northern subcontinent, especially among Brahmin, Bhumihar, Ror, Jat, and Kalash communities.
Steppe-derived populations, including the Yamnaya, are thought to have brought lactase persistence into Europe from the Eurasian steppe. Five ancient DNA samples from Yamnaya sites showed a frequency of over 25% for the associated allele, and it is hypothesized that this gave steppe migrants a biological advantage over resident European populations who lacked it.
Around 90% of Britain's gene pool was replaced within a few hundred years following the arrival of Bell Beaker populations carrying steppe ancestry. The genetic turnover in Britain was the most substantial of any region documented in ancient DNA research on this migration period.