PPS: My post at 2021 refers to the way that things seem to be done with most parking apps and in most situations. (I certainly see ticket inspectors checking on vehicles in the council car parks, where such apps are in use, in Ipswich and Stowmarket).
However it's also possible for such apps to link in with the ANPR cameras which are in use in some car parks. For example, Ipswich Hospital car park no longer has ticket inspectors. People without the app pay at the end of their stay (by cash or card), with the duration of that stay being calculated based upon the entry time of their vehicle (as detected by the cameras at the car park entrance). Those with the app on their phone though just pay as they would on any other car park, with the 'in' and 'out' cameras checking that they've paid the correct amount for the duration of their stay.