So in that case, the order of the transactions is already established, and that is the order in which the transactions must be confirmed. The transaction that depends on another must confirm at the same time or after the transaction that it is spending from.

Before a transaction is included in a block, the transaction is stored in the memory of nodes (including the miner's node) in a place known as the mempool. When a miner goes to choose transactions for a block, he will select transactions from his own mempool. Once the block is broadcast, nodes will then remove the transactions in those blocks from their own mempools.

Transactions in the mempool are unconfirmed. They are not permanent. They are not in the blockchain. Not every node will necessarily know about all of the unconfirmed transactions, nor do they have to.

When a transaction is broadcast, it first goes to the mempools of the nodes. If it remains unconfirmed for a long time, some nodes may choose to drop the transaction from their mempool to free up some memory. Other nodes may choose to rebroadcast the transaction so that it remains in the mempools of as many nodes as possible (i.e. those nodes are reminded of the transaction if they dropped it). Other nodes may simply keep it there until it confirms or the node reboots and the mempool is cleared.

The blockchain is the public ledger. The miners who finds the next block for the blockchain is the one who establishes what is going into that ledger.

Miners can choose whatever transactions that they want to include in their block. So long as the block is valid, it will be accepted by everyone else.

There can be multiple valid blocks for a given block height. This happens fairly often. The blocks which are not built upon are called stale blocks.