When two miners solve the Proof-of-Work on the same block at the same time, it can create two branches of blocks on the blockchain. In order to ensure that only one branch remains, the Bitcoin network adopts a policy of longest chain wins. This means that the branch with the most blocks is accepted as the true version of the blockchain and the shorter branch is discarded.
Before discarding the shorter branch, all the transactions in the blocks are returned to the transaction pool so that they can be mined and added to some future block. This allows the transactions on the shorter branch to eventually be added to the blockchain, ensuring that no valid transactions are lost.
By using the longest chain wins policy, the Bitcoin network is able to maintain a single version of the blockchain and resolve any conflicts that arise between miners. This ensures that all transactions are properly recorded on the blockchain and that the blockchain remains immutable and secure.