Many, many threads on this topic, for both the iPhone and iPad. Here is but one of them: iOS 18 Calendar app hangs regularly with … - Apple Community
TLDR: put your phone into airplane mode, ensuring that Wi-Fi is also turned off. Then restart your phone, go into control center, and remove your exchange accounts from calendar (I actually want up removing all the calendar accounts). Next delete your calendar app (yes, really). Then download Microsoft Outlook for iOS (I know), and use that for your calendar until Apple fixes this.
I went so far to stop using and deleting Mail too, because I didn’t want to find out it would be buggy as well. This stopped the freezes and battery drains I experienced as I attempted piecemeal solutions.p (e.g. just removing calendar accounts and not the app itself).