Which format does Android calendar uses? I have a calendar entry on the format of (iCalendar with extension: ics) I have downloaded this file but android does not understands this kind of file.

Is there alternative to iCalendar files that Android uses?
Due to how file extensions handling works in Android, it depends on the calendar apps themselves.
For example, AOSP Calendar does not handle any file extensions. The manifest file only mentions filters to the content provider and Google Calendar websites.
&lt;intent-filter&gt;
  &lt;action android:name=&quot;android.intent.action.VIEW&quot; /&gt;
  &lt;category android:name=&quot;android.intent.category.DEFAULT&quot; /&gt;
  &lt;data android:mimeType=&quot;time/epoch&quot; /&gt;
  &lt;data android:host=&quot;com.android.calendar&quot; /&gt;
  &lt;data android:scheme=&quot;content&quot;/&gt;
&lt;/intent-filter&gt;

...

&lt;intent-filter android:priority=&quot;50&quot;&gt;
  &lt;action android:name=&quot;android.intent.action.VIEW&quot; /&gt;
  &lt;category android:name=&quot;android.intent.category.DEFAULT&quot; /&gt;
  &lt;category android:name=&quot;android.intent.category.BROWSABLE&quot; /&gt;
  &lt;data android:scheme=&quot;http&quot;  android:host=&quot;www.google.com&quot; android:pathPrefix=&quot;/calendar/event&quot; /&gt;
  &lt;data android:scheme=&quot;https&quot; android:host=&quot;www.google.com&quot; android:pathPrefix=&quot;/calendar/event&quot; /&gt;
  &lt;data android:scheme=&quot;http&quot;  android:host=&quot;www.google.com&quot; android:pathPattern=&quot;/calendar/hosted/.*/event&quot; /&gt;
  &lt;data android:scheme=&quot;https&quot; android:host=&quot;www.google.com&quot; android:pathPattern=&quot;/calendar/hosted/.*/event&quot; /&gt;
&lt;/intent-filter&gt;

Some vendor and 3rd-party apps may handle some file formats, but without knowing their manifest file, one has to manually test with each file format.
The calendar data itself is usually stored in the Calendar Storage (or similar name) app, in the format of SQLite database that can be accessed with the Calendar Provider. Thus, 3rd-party apps can also import the data from files to the calendar storage, which then can be read by the calendar apps.