Activitiy's email content - how is it generated and can it be changed?
I think the template you're looking for can be found at Administer -&gt; CiviMail -&gt; Message Templates, i.e, navigate to civicrm/admin/messageTemplates?reset=1 and click on System Workflow Messages.

The template name is Cases - Send Copy of an Activity. Note that you don't need to enable CiviCase component to view this template. It is displayed by default and used to send emails to assignees(both case and normal activities).

Also, if you want to disable the sending of email to assignee for some activity types, there is a new configuration setting added at civicrm/admin/setting/preferences/display?reset=1. 



The setting is only displayed when the above checkbox of Notify Activity Assignee is enabled.