Complex sentence: He built missions extending from present day states of Mexican Sonora, northeast for, into present-day Arizona, where the San Xavier del Bac mission, near Tucson, a popular National Historic Landmark, is still a functioning Franciscan parish church.
Split into simpler sentences: He built missions extending from present day states of Mexican Sonora, northeast for, into present-day Arizona. There, the Mission San Xavier del Bac near Tucson, a popular National Historic Landmark, is still functioning as a Franciscan Catholic parish church.