My friend's A71 got one UI 5.1 update few days ago, but my A72 still on one ui 5. Not getting update even my friend's down model got it. What nonsense?

please any one tell me what is difference between unlocked phone and other phone??? i want to buy s9 new phone. when i cheaked imformation from internet. there is too much type. please tell me which phone is best. unlocked or other??? and what mean b...