Adding new default components to the database
A bit of terminology: Components in CiviCRM usually means CiviContribute, CiviEvent, CiviMembership, etc. I think you are referring to parts of an email template when you write about components. In general, when you are wanting to add something to the database in CiviCRM as part of the core code, you need to make sure it is added to the install tarball and is added or changed in databases as they are upgraded. 

To insert data in the tarball, look under https://github.com/civicrm/civicrm-core/tree/master/xml/templates - in your case you likely want to add or update templates in https://github.com/civicrm/civicrm-core/tree/master/xml/templates/message_templates

To insert or modify data on upgrade, look at https://github.com/civicrm/civicrm-core/tree/master/CRM/Upgrade. Note that the message templates are in subdirectories by release, and that non-message template insertions are generally in the per release upgrade scripts in https://github.com/civicrm/civicrm-core/tree/master/CRM/Upgrade/Incremental/sql