Create event sources 

A calendar can use any number of event sources at the same time. The records of the feeds are stored on a page (e.g. a folder General storage), the events of EXT:calendarize are selected by their storage pages.

iCal feed 

Create a record iCal Feed in a folder and fill in the fields.

The iCal feed record with title, URL, color and CSS classes
Title
Name of the feed.
Feed Url
URL of the .ics file. A path relative to the web root (e.g. /fileadmin/calendars/holidays.ics) works as well as a complete URL.
Color
Background color of the events of this feed. If empty, the color of the first category of the feed is used.
CSS classes
Additional CSS classes, added to every event of the feed.
Categories
Categories of the feed. They are added to the events as CSS classes and are used by the filter.

The tab Access allows to show a feed only to some frontend user groups.

Google calendar feed 

Create a record Google Calendar Feed.

The Google calendar feed record with title, API key, calendar ID and color
Google calendar api key
An API key of the Google Developer Console with the Google Calendar API enabled, see Google calendar: API key and public calendar.
Google calendar id
The ID of the calendar, found in the settings of the calendar in the Google Calendar web interface (section Integrate calendar).

The fields title, color, CSS classes, categories and access are the same as for the iCal feed.

EXT:calendarize 

If EXT:calendarize is installed, the storage pages of its events are selected inside of the plugin (tab Event sources). No feed record is needed. The categories of the calendarize events (system categories) work as categories of the calendar.