Where does Mosaico store my templates?
The templates are stored in your CiviCRM database in the table civicrm_mosaico_msg_template. They are not saved as actual files. BTW Im assuming you are using the Mosaico extension?
If you go to CiviCRM->Mailings->Mosiaco Templates you get full options for tweaking existing templates, copying them and so on. See attached image.