A Bitcoin transaction begins as a message signed by a private key. It says which previous outputs are being spent, where the value is going, and how much is being left behind as a fee.
The wallet broadcasts that message to a node, which checks it against the rules — are the inputs real, unspent, and correctly signed? — and then relays it to its peers. Within seconds it is sitting in the mempool of nodes all over the world.
Miners select from the mempool, generally favouring the highest fee per byte, and assemble a candidate block. When one of them finds a hash below the current target, the block is published and the transaction is confirmed once.
Each subsequent block makes reversing that history more expensive. This is why exchanges wait for several confirmations on large deposits: not because the first one is fake, but because depth is what makes it costly to undo.
