The word 'Uska' in hindi means her/his/its. When clicked the word for meaning, it also showed he /she /it/that has..
So when is 'uska' used and when is 'vah/yah' used?
Uska or उसका means his/her/its. For example, 'his house' means 'उसका घर', 'her apple' means 'उसका सेब'.
'वह/यह' or 'vah/yah' means he/she/it. For example, 'वह खाता है' means ' he/she eats'.