Complex sentence: He began his trapping career with a Frenchman named Robideau headquartered in St. Louis, then spent several years in the Oregon Country working for the Hudson's Bay Company (HBC).
Split into simpler sentences: He began his trapping career with a Frenchman named Robideau who made his headquarters in St. Louis. After this he spent several years spent in Oregon Country working for the Hudson's Bay Company (HBC).