In the Calendar view, you can click on the iCAL button to generate a link that can be used to add calendars to any external calendar application, like Google Calendar:
In the following pop-up, single or multiple calendars can be selected for export:
If more than one calendar is exported simultaneously, they are grouped together, such that the events of the original different calendars are no longer distinguishable. For example, you could create a calendar for all absences. More information on this topic can be found here.
Then copy the generated link to add the calendar to an external application.
The exported calendar is automatically synchronized by your calendar application via the link provided. It will periodically update to the current state of the calendar at Personio.