Complex sentence: During the 1980's and the 1990's, a large number of high-skilled Hong Kong immigrants settled in the San Gabriel Valley of Los Angeles County, and the San Francisco Bay Area, where many were employed by high-technology companies in Silicon Valley.
Split into simpler sentences: During the 1980's and the 1990's, many high-skilled Hong Kong immigrants settled in the United States. Many of them moved to the San Gabriel Valley of Los Angeles County, and the San Francisco Bay Area where many were employed by high-technology companies in Silicon Valley.