My understand, for a tx, before it finalized with the locktime, it can not go into mempool. But, which specified location is the tx to stay for waiting the locktime come. Similiar, for before the sequence number, where is the tx to stay for waiting? Thanks.
The tx is located with the person who created it. Nodes will not hold onto the transaction because it is not ready for the mempool. The only places a node will hold someone else's transaction is in the mempool or in the blockchain so a node will not store a non-final transaction anywhere.
It is up to the transaction creator to hold onto the transaction and not lose it until they can broadcast it after the lock time has passed.