Hire creates a worker and connects the person to a job, position, manager and supervisory organization. Those relationships must agree before you submit. In a live tenant, additional fields and approvals depend on the organization's staffing model and configuration. This practice tenant uses a short manager and HR Partner approval chain.
A submitted hire isn't yet a completed hire. Open the Inbox, review the assigned steps and finish them in order. Then inspect the new worker record and the process history. The effective date says when the hire applies; the submission date says when you initiated the transaction. Choose an open position rather than overwriting the incumbent of a filled position.