how do I transfer all my files from QB2012 to QB2019?
If QB2019 will not directly open or restore from your QB2012 file you may need at least one additional interim upgrade. You will NOT find such an animal available for free download nor would you be able to activate a copy of QB2015-2016 for example as they are long past being supported.
Our client has converted their QB 2013 data file on QB 2020 in one jump. You should try doing the same. Open your 2012 data file on your QB 2019. If you encountered any error, you may convert it on QB 2016/2017, or purchase a 3rd party conversion service.