how does 'starter ' added to milk help it to set into curd ?

Starter added to fresh milk to set into curd is a small amount of curd that act as source of millions of Lactic Acid Bacteria (Lactobacillus). These millions of LAB multiply at suitable temperature and produce lactic acids. The acidic medium produced because of lactic acid oagulates the milk proteins like casein, converting milk to curd. Lactic acid is produced through fermentation of lactose.