Intercept and edit transaction output When a P2PKH transaction is created there are two main parts: the transaction input and the transaction output. I understand that the transaction input requires a signature that unlocks a previous UTXO for that specific address.

So in simple terms we have 

transaction = transaction input(UTXO+sig) + transaction output(destination address and amount)


What prevents someone from intercepting the transaction and replacing the 'transaction output' with their own address?
The sig part signs the entire transaction, not just the input part. So if someone intercepting the transaction replaced the transaction output part, the sig would no longer be valid. In fact, changing other inputs or outputs or amounts would also render the sig and thus the transaction invalid.

When you sign away an input, you are essentially saying "only this very specific transaction may spend this input".