Where is a tx located before finalized with the sequence number or locktime?
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.