This applies to activities attached through Calendar Activity Mapping, which is only available on the new candidate profile.
Once a custom activity is attached to an event, the two stay visibly linked wherever you look at either one.
On the Activity Card
The custom activity appears on the candidate's timeline like any other logged activity, labeled with its activity type. Because it was created through Calendar Activity Mapping, it also shows a reference back to the event it came from.
On the event details
Opening the mapped event, from the calendar or the event list, shows a reference to the linked custom activity, so you can jump to it without hunting through the timeline.
Things to know
The link is visible in both directions: from the event to the activity, and from the activity to the event.
If you don't see the linked reference on either side, see Troubleshooting: Custom Activity Not Attached or Not Showing on a Calendar Event.