If transactions are stored in Bitcoin blocks through Merkle trees, when the last Bitcoin block is mined, how would there be future Bitcoin transactions? Where would those transactions be stored on?
A misunderstanding. I do not know what leads you to believe that there will be a last block.
It is true that block rewards have a halving period and will eventually, in many years, reach zero, but, the transaction fees are also paid to the miner so mining will continue.