You have a spendable amount of bitcoin and only want to send part of it. What happens to the rest?

Detailed explanation

When a wallet selects a UTXO as an input, that UTXO is spent in full. The wallet cannot simply cut the desired payment amount out of the old UTXO. Instead, the new transaction allocates the full input value among new outputs and the network fee.

If the selected value is larger than the payment plus the fee, the wallet normally creates a change output. That new output is again controlled by the user's wallet and can later be spent as another UTXO. The remainder therefore does not stay inside the old UTXO.

The amount of change depends on the selected inputs, the payment amount and the fee. Wallets often use a new internal address for change. This reduces direct address reuse but does not automatically make the transaction anonymous, because amounts and later spending can still provide clues.