tony montana

I am learning about oxymercuration-demercuration reactions with alkenes and I wanted to know why this reaction occur? I mean why would oxymercuration-deoxymercuration occur compared to it not occurring? Same with hyrdroboration-oxidation. I mean why doesn't direct hyrdration just occur

Why in some situations the Hydrogen is the nucleophile and in others the Bromide? In other words, how can you tell what the nucleophile is in the problem?

I am trying to understand how and why these reactions differ. I know that the nucleophile should always attach to the less substituted carbon of a double bond in an addition alkene reaction. However, in my textbook, there are two example questions that are confusing me. One example shows a hydrogen as the nucleophile and the other example shows Bromide as the nucleophile. Would this have anything to do with the fact that the Bromide came from a free-radical?