Clients can choose to pay freelancers hourly or by the project. If it's a fixed-priced project, then you or the client can determine how your work will be divided up and submitted. These are referred to as milestones on the Upwork freelance site.
Milestones are used to break up larger projects into smaller deliverables. Clients can set up payments for individual milestones and check work in different stints as the project progresses.