There's nothing in the sentences themselves that indicate they need to be in past perfect. You need context to know this. For example with #4 you could say 'Puja won a big prize' and that's perfectly correct. You use past perfect when there's another event I.e. Puja had won a big prize before he decided to leave the fair. In your sentences the other event, or interruption is implied, not stated. They're not very good examples for explaining how to use the past perfect. You could also use present perfect I.e. Pooja has won a big prize. The implication here is some sort of effect on the present moment. Perfect aspect can be hard to get coming from languages that don't have it. Message me if you're still not sure.

We use the past perfect to talk about what happened before a point in the past. It looks back from a point in the past to further in the past. e.g I checked with the supplier and they still hadn't received the contract.

We can discount present-perfect because of the word 'had' (past tense form of 'to have', obviously, to which the word 'had' would have to be changed). Then, we can discount the past (simple) because of the presence of the auxiliary verb 'had'. Simple past is just 'they finished', 'I learned', 'Amar made' and 'Pooja won'. It's difficult to make the past and past perfect (or rather pluperfect) distinct temporally since the sentences are so simple, but their usage and ability to define timing of an action is different (that is, past tense can only do so with respect to non-past tense, while past perfect can also do so with respect to continuous and simple past). In day to day conversation, it's usually a moot point (and often contextual based on surrounding conversation), and found mostly in prose written almost entirely in the past tense.