Having that payment code addresses are untracable, how does a transaction look like in a block explorer if someone sends a payment to a reusable payment code?
BIP47 transactions look like any other transactions as far as inputs/outputs & fees are concerned. There is no difference.
Payments are not sent to the payment code itself but rather to addresses generated by the shared secret of both the sender's payment code and the receiver's payment code.
The only recognisable trace on the blockchain is the notification transaction, of one payment code to another, which includes an OP_RETURN. Notification transactions are done once for a sender to a receiver to initialise the payment "channel" and also permit any future restore of the wallet directly from the blockchain (in the event of lost metadata, for example).