You can read how Quorum achieves privacy in the wiki on this page: https://github.com/jpmorganchase/quorum/wiki/Transaction-Processing.
I'll explain how this works here but not as comprehensively as the link does. Essentially, key to Quorum's privacy features are 2 fold:
- Quorum implements 2 states: public and private. Private state is maintained at the node level and reflects changes to private smart contracts that the node was party to during the lifetime of the blockchain. Private smart contracts are smart contracts that are only visible to N number of participants.
- Private transactions that a node executes have its payload data stripped and shared only with the intended recipients of the payload. The public chain records a txn which is hash of the encrypted txn shared between the parties (txn is written with special v value) and the actual payload is encrypted and passed over another layer through a private transaction manager (constellation / tessera)
Call me biased, but I do happen to think that Quorum is the best choice for privacy handling that is as close to Ethereum as possible, after all, Quorum is a fork of go-ethereum client. As for use cases: as long as one needs a private blockchain, Quorum will fit and support any use cases applicable to Ethereum. Privacy is only one of the major features, but as most features, op decides what to use in their use case. The other features are several supported consensus algorithms, network level permissioning, and block finality.