Complex sentence: They are the Uruguayan descendants of colonists from Spain and Portugal during the colonial period prior to 1810, and mainly of immigrants from Europe in the great immigratory wave during the late 19th century and early 20th century.
Split into simpler sentences: They are the Uruguayan descendants of colonists from Spain and also from Portugal during the colonial period prior to 1810. The majority of immigrants from Europe arrived in the great migratory wave during the late 19th century and early 20th century.