In Simple Words
Bitcoin is not a file that moves from one phone to another.
Your wallet creates a transaction that specifies the destination address, the amount you want to send and the network fee.
Your private keys then create a digital signature. This proves that you are authorized to spend the bitcoin without revealing the keys themselves.
The signed transaction is broadcast to the Bitcoin network, where nodes check that it follows the rules.
When a miner includes it in a block, it receives its first confirmation. In simple words: the wallet creates the transaction, your keys sign it, and the network confirms it.