Agenda/ Calender: How can I put a calendar item in multiple calendars at the same time?

It gets stored only in the Default Calendar set in Settings > Calendar > Default Calendar.

Each Calendar is linked to an email account and each account has an email ID e.g.

Gmail Calendar:

iCloud Calendar 1:

iCloud Calendar 2: (Say Default)

Yahoo Calendar:

The process to add a Calendar event on multiple calendars at the same time.

1. Open your iPhone's Calendar app.
2. Add an event and this event gets added to the iCloud Account 2, This is set as default in our example.
3. During the addition of an event or If you have already added the event, open the event and Tap "Edit" in the top right corner of the Event Details page.
4. Tap "Invitees."
5. Add the email account ID of Gmail, iCloud 1, Yahoo. (This action will add the event in all the calendars)
6. You will get 3 Popups
7. Accept Invitation.

Hurray!

Where were the calendar events stored?

I am not sure where you see Agenda in an iPhone Apple Calendar --> Create and edit events in Calendar on iPhone - Apple Support