What is usually needed to get both a work calendar and a personal calendar to show in the same place on a mobile device?

A plug-in

An add-in

An app

Permission sharing

Permission sharing

Explanation:

As each calendar represents a set of data that has Share preferences, you can switch it from being isolated on a desktop computer let's say, to shared on a mobile device. Since these two are not separate programs or apps, no plug-ins are needed. Also, this is not an add-in functionality. All you need to do is set the Share preferences on each calendar. Then they will become integrated on the selected device.