Hi @Felicia @itsellageee ,
You don’t have to have an exact UTXO to send a transaction. Let me use your example to explain it. If you have a UTXO with 1btc and you needed to send 0.6 btc you will be making a transaction with 1 input and 2 outputs. Since you can’t simply leave 0.4btc and send 0.6btc, you must put that 1btc UTXO as the input. The first output will be 0.6btc, which is going to be the address you originally wanted to send to. The second output will be 0.39 (0.01 fees for the miners) back to an address that you own. This will all be done by the wallet automatically.
To answer your question, no. The balance will be send back to you, to a new address that the wallet just created. (Generate random private key, get public key, get pubic address <-- Send the remaining funds back here)
Yes, increasing the amount of either inputs or outputs in the transaction will increase your total fee. Let’s say the current average fee is 2 satoshi per byte. If you have 1 input and 2 outputs your transaction would be lower in size compared to a transaction with 1 input and 10 outputs. And since we pay satoshi based on the amount of bytes taken, it is natural that larger transaction would have to pay more. That’s why we use the unit of measurement “satoshi per byte”.
The wallet is the one that creates the transaction. If you use a malicious wallet, the bitcoin network will accept the transaction nevertheless. As long as it is valid. In the bitcoin network there is only valid and invalid transactions. But yes, your understanding is correct.