The problem that you describe likely being very easy to resolve.
It is highly probable that an inadvertent click on a website link has “subscribed” an additional (unwanted) Calendar to your device - and this unexpected Calendar is exposing unwanted calendar events and sending you unexpected “adverts” or other warnings.
Let’s check for what’s out of place...
iOS/iPadOS13 and earlier: Settings > Passwords and Accounts
iOS/iPadOS14: Settings > Calendar > Accounts
Look for an “account” that shouldn’t be in the list of accounts - as this will likely include the Calendar that contains all the unwanted events. When you find the suspect account, tap - then select Delete Account. This should resolve the problem in its entirety.
I hope you find this helpful in finding the source - and in resolving the issue if an unexpected Calendar is found.