What happens when tx is broadcasted whose input is not yet received by mempool or are unknown?
Such a transaction is called an orphan transaction.

When you announce a transaction to a peer who misses one or more of its inputs, the transaction will be placed in the orphan pool - where they are temporarily held in the hope that their dependencies arrive. A request will go out to fetch those dependencies from whoever sent us the transaction.

The transaction won't be relayed until it can be validated. Once all dependencies arrive, they will all be subject to normal validation and relay policies, and if acceptable, relayed in dependency order.