By default, an employee's manager will receive time off requests. If that employee doesn't have a manager the company owner will receive their requests. Admins will have their requests approved automatically so no one is notified. Read more about Air's default time off notifications here.
To customise who approves time off:
You have full control over who is notified and approves time off requests for each user in your company.
Go to the employee you'd like to configure
Select the Time Off tab
On the Overview tab, underneath their balances you'll see Time Off Approvers.
Click Choose Approvers to select who should be notified and can approve or decline this user's requests. You can also select Auto Approval and no one will be notified. You'll only be able to see ACTIVE users to set as approvers. You cannot set inactive or not-invited users as approvers.
Note: Each "Approver" you select will be notified of new requests and can approve or decline them. Requests can only be approved or declined one time, so once someone has reviewed a request any further reviews won't have any effect.