Periods live under Payroll and each person's history on their record; resends go from the individual payroll into the audit trail. Terminated staff kept 5 years.
Where to find them
Periods are listed under Payroll, newest first, and inside each one is every employee's payroll. The employee record shows their history across all periods, which answers "how much did they get in March" without opening period by period.
Resending
Open the individual payroll and use the email action. The resend produces the same payslip (the approved calculation does not change) and a new audit-trail entry, so you can prove the document was delivered again.
Retention
A terminated employee's payroll stays available for 5 years after termination (the default retention policy), after which their personal data is anonymized. Before that, you can export all of the employee's data from their record.