2/3. "PayJoin (also called pay-to-end-point or P2EP) is a special type of CoinJoin between two parties where one party pays the other. This coinjoin type has different (probably better) privacy properties. The transaction then doesn't have the distinctive multiple outputs with the same value, and so is not obviously visible as an equal-output CoinJoin."

Payjoin is a batched payment and consolidation transaction between sender and receiver. The result has some fee savings from batching and privacy benefits from the joint structure. Both sender and receiver contribute inputs and may create outputs. The result can look the same as normal transaction activity thus creates more indistinguishable transaction interpretations for all. Nothing limits payjoin to 2-input 2-output transactions.

What's nice about PayJoin too is that because it is effectively a two-person coinjoin, someone who is sending normally can make their transaction look like a payjoin when really it was just a normal send. It gives probably privacy to everyone on the network bc you can't know whether a transaction invovled 2 people or just 1.