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How did Yiddish develop from High German dialects?

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Thematic map of Europe displaying the distribution of Germanic languages, with different colors indicating regions where English, German, Netherlandic (Dutch), Frisian, Swedish, Danish, Norwegian, Icelandic, and Faroese are spoken. Striped areas mark regions where bilingualism is common, and language regions cover countries such as the United Kingdom, Germany, the Netherlands, Denmark, Sweden, Norway, Iceland, and parts of surrounding countries. — West Germanic languages Distribution of the Germanic languages in Europe.

Yiddish originated in the 9th century as the Ashkenazim emerged as a distinct cultural entity in central Europe. The language developed through an intricate fusion of two primary linguistic stocks: a Semitic component (consisting of postclassical Hebrew and Aramaic) and a grammatically and lexically potent Germanic component.1

This Germanic foundation was gleaned from a variety of High German and Middle German dialects. While Yiddish is a Germanic language, it is not a typical one; it evolved through continuous contact with other languages, including Romance languages early on, and later Slavic languages as the population shifted eastward.12

Because of these diverse influences, no single Yiddish dialect stands in a one-to-one relationship with any specific German dialect. Over time, Yiddish integrated these elements in unique ways, such as using German-derived word elements within Slavic grammatical models or combining borrowed vocabulary in manners unfamiliar to the source languages.23