In such a case, the payment method itself, when validated, is the proof-of-payment, with the actual journey detail not yet known.
Instead, you just tap on and off,[1] and the system will automatically charge you the correct amount, without an actual physical ticket.
This was a predecessor to stored value cards, developed to be a more durable replacement of magnetic tickets.
Although it's a one-time purchase for residents, visitors still need to buy a card specific to the region, and it can be hard to get any remaining value back after leaving.
During the 2010s, public transport providers start to develop contactless payment as an alternative to stored value card.