I have the two most commonly used by our closest towns. I’ve stored details of my car registration and a credit card on them.
When I am in the car park I look to see which app I need, open it, it asks me to confirm I’m in car reg whathaveyou and using credit card ending on n such and such four digits. I confirm, tap in the parking area reference number, and the amount of parking time I require.
The app adds 10 or 20p to the parking fee, charges my card, and off I go. It sends me an alert when I am near the end of my time, so I can decide whether to pay more or return to the car.
I use a third online company, but it’s only for a rarely used car park, so I just store their number in my phone, they have my car reg and card details, and I phone to buy the parking.