Fees on Bitcoin are via transaction which is derived via the UTXO. So if I put 5BTC as inputs, and 4.9btc as outputs, the 0.1BTC not mentioned are fees. For me to add more fees to that transaction or get the 0.1BTC back, I need to add 0.1BTC to a change address that comes back to me. That means that I’m changing the original transaction and therefore I have to resign the inputs.

Ethereum is account based, not UTXO