When you try and send a book to a device, then Calibre will first check to see if it has it any of the formats configured as being supported for that device type. If it finds any then the highest priority one in the configured list is sent. If no suitable format is found then a conversion is done to the preferred format for that device before sending the book.
If you are using the calibre standard 'Add Books' facility from the menu bar there is no automatic conversion - in this case conversions are manually initiated.
If you are using the facility for automatically adding books from a watched folder, then you can specify that a conversion should be done (if required) to your preferred format as part of the add process.