This is very very helpful. I did not know blockchain works like that. It's not like I have an address A with 1 BTC and I want to send 0.5BTC to address B, so after that address A will be left with 0.5BTC, what actually happens is that address A is left with 0BTC and 0.5BTC goes to fee
I would recommend having a read of this page:
https://learnmeabitcoin.com/beginners/outputs. It very clearly breaks down the concepts I described above, that you can only spend an entire output (or multiple outputs), if you don't want to spend the full value to the other party then you need to specify to send some back to yourself as change, and whatever is left over that you haven't specified a destination for gets used as the fee.
I would also suggest, though, that since you don't have a firm grasp yet on how outputs and transactions work, that you should use a client such as Electrum which will do all this for you in the background, rather than trying to build a transaction manually and ending up losing large amounts of coins.
That's why I'm coding, it's easier to learn when I actually work on it and building it