Your post title doesn't seem to align with what you're asking? In any case ...
For using the stock Calendar app on your iPhone:
Did you create the Exchange account(s) in your Settings? (Settings/ Passwords & Accounts/ Add Account/ Exchange) That would be the first step, and then during the process of adding the account you elect to enable Calendars for the account.
Once you've done that, open the Calendars app and at the bottom-center tap "Calendars" to show which ones have been enabled for viewing -- each account may have more than one calendar. Select the appropriate calendars for what you want to see.
Synchronization should occur with little delay, and should do so over Wi-Fi or cellular. However, administration of the Exchange account, and their policies and procedures, may impact synchronization so you may need administrator assistance if this is a synchronization issue.
If you're wanting to use the Outlook app for Calendars, refer to Microsoft's instructions on how to set it up.
Does any of that help? If not, maybe explain with clarity what you've already done and what exact symptoms you're seeing.