I'm a novice and had this thought. I understand that data in an existing block can't be altered, as it changes the hash. But what if, before creating the block, I take transactions from the pool, alter one transaction, and then proceed to create the block? Let's say I have the most powerful hardware and finish it before other miners. Will other nodes be able to verify the transaction data inside the block? (example: say A sends 10 tokens to B, and I change that to 100. A has 200 tokens in his wallet)
You can modify it, but in doing so, you will invalidate the signature on the transaction, since the data is no longer the same as what was signed.
You could do so on transactions that don't involve signatures, but they are generally non-standard on the mainnet.
Note that even after modification, you can only update it within the bounds of the input(s) sum. You can never modify a transaction to have an output greater than the sum of the inputs.