Please help...How does the shudra make up the indian caste system

Shudra, also spelled Sudra, Sanskrit Śūdra, the fourth and lowest of the traditional varnas, or social classes, of India, traditionally artisans and labourers. The term does not appear in the earliest Vedic literature. Unlike the members of the three dvija (“twice-born”) varnas—Brahmans (priests and teachers), Kshatriya (nobles and warriors), and Vaishya (merchants)—Shudras are not permitted to perform the upanayana, the initiatory rite into the study of the Vedas (earliest sacred literature of India).