Hi there, wolfkat98!
Thanks for reaching out to the Apple Support Communities! It sounds like you're having an issue viewing shared calendars from an Exchange account in the Calendar app on your iPhone. According to the Microsoft website, this may not be possible unless you're using the Outlook for iOS app and Microsoft 365. You may be able to find help in this link for Calendar sharing in Microsoft 365 - Microsoft:
The third improvement is that users can view all their shared calendars on any device or calendar application. If someone changes a shared calendar, all other people sharing the calendar will see the changes instantly. Users will also notice performance improvements when viewing and editing shared calendars. Using a shared calendar will be as fast and reliable as managing the user's own calendar."
If more help is needed, we recommend reaching out to Microsoft Support for additional assistance.
Cheers!