On Chapter 6 - Transactions of Mastering Bitcoin it is said:
Note that each transaction input is signed independently. This is critical, as neither the signatures nor the inputs have to belong to or be applied by the same "owners." In fact, a specific transaction scheme called "CoinJoin" uses this fact to create multi-party transactions for privacy.
Each transaction input and any signature it may contain is completely independent of any other input or signature. Multiple parties can collaborate to construct transactions and sign only one input each.
I understand the fact that all signatures are independent from each other and therefore can be computed separately, but I always had the impression that the signatures are calculated from the transaction hash, and therefore this would make them dependent on the inputs, meaning one would need to know all the inputs included on the transaction in order to generate the correct signature.
If my understanding is correct, I believe a better wording of this statement should be something like:
the signatures needed to spend each UTXO referenced by each transaction input are computed independently
What did I got wrong here?