Hello,
Welcome to the QuickBooks community. I'll be happy to assist with this. QuickBooks Online offers a great platform where you can manage your business from the office and on the go. That being said, the mobile app doesn't give access to all the available features of the program. Certain functionalities, including the payroll service, are not available through the mobile app. That's why you won't be able to see the employees list on iPhone. If you wish to access the payroll tab and manage employees, I suggest logging in through your browser.
To know more about the QuickBooks mobile app features, check the official page here.
Feel free to reach out if you have any other questions.