The fixed price basis is a work model that assumes that the final price is generated before the start of the work, and it cannot be changed during the work process. However, the disadvantage of this model is that the app requirements are fixed as well, and they cannot be modified during the work.
Hourly rate basis is the work model when the client pays for the actual working hours of developers on a weekly basis. It is a flexible model, and it's possible to change app requirements at any time. But the disadvantage of this model is that nobody knows when the project will be finished & what the final price will be.