but why the digestion of starch starts in buccal cavity and ends in stomach only ?

Digestion starts in mouth as it secretes saliva which contains amylase enzyme necessary for digestion of starch. The digestion of starch stops in stomach as gastric juice secreted by stomach does not contain amylase enzyme and the enzymes of saliva are deactivated by acidic medium in stomach. The digestion does not stop here but the partially digested food then travels to small intestines where complete digestion of starch takes place by the enzymes present in pancreatic juice and intestinal juice.