Question

how the substrate changes into digestive product​

Answer 1

The enzyme serves as the lock and the attracted molecule (called the substrate) is the key. Once the chemical reaction within this lock and key arrangement has been completed, the products are released and the enzyme is free to attract another substrate molecule.

Answer 2

An enzyme's active site and its substrate are complementary in shape. An enzyme will only work on one substrate - it is substrate specific. Enzymes and substrates collide to form enzyme-substrate complexes. The substrates are broken down (or in some cases built up). The products are released.