I hope you're just referring to the bus routes in Edinburgh!
The payment system stinks: cash-wise, they only accept exact change only. You can pay a £1.70 single with £2, you just don't get change.
You can pay with the bus app, but this is a hideous payment app that is seriously flawed. Firstly, you have to top up minimum of £10, so no automatic integration to your bank account or PayPal. Secondly, you have to 'purchase' a ticket on the app just before the bus arrives, and it has a time limit. Within this time-limit of 2 minutes, you have to show the driver the ticket. The driver does nothing other than Yay/Nay it. No scanning or anything. Makes it infuriating to use. This is not just Edinburgh, but other parts of Scotland, like Dundee.