Welcome to the QuickBooks Community. I'll share information about the payroll service and QuickBooks Online (QBO).
Intuit offers a standalone QBO Payroll and one that is an add-on to QBO. The standalone payroll is considered a different product and cannot be synced or integrated into QBO.
If you need references for QBO Payroll, just click on Help and search for the topics you like. Or, you can visit the QuickBooks Support page, and select the QuickBooks Online Payroll product.
I'm just around if you need more assistance with payroll. Take care and have a good one!