Calendar

If you enter an event in multiple calendars you will therefore have duplicate events, which, in my view anyway, will become a mess fairly quickly. You don't need multiple calendars in the app necessarily, but most users find it useful to separate their events into categories of similar events.

You might have one for bills payments, one for social engagements, one for sporting activities, one for work meetings, and so on. My family's Calendar app has well over a dozen we share, including one just for meal planning.

Separating your events then gives you the ability to turn off the display of all the events in that calendar. This is particularly useful if you need to refer to those events periodically; school events might be crucial for the child, but might clutter up a parent's app where a family shares their iCloud calendars.

You can also set up a calendar for a short period, perhaps for a work project or a holiday, then delete it later when it isn't required and leaving all your other calendars untouched.

Calendar seems such a simple app - Apple designed it to be easy to use - but to get the most out of it takes a bit of learning. Fortunately Apple provides a very useful Calendar User Guide for Mac. Most of this also applies to iOS devices.

Set up multiple calendars on iPhone is also useful.

But to return to your question, if you need to have one event in multiple calendars you will have to create it again in each calendar that you wish it to populate.