1- depend of the OS and the folder, you can already have one.
2- yep it’s a text file, called what_you_want.json with this contain
3- you can remove it before the inclusion, will prevent conflict with entry with legacy code and future one with DDF core, generaly a simple re-inclusion is enought, but I have stranges issues ATM, so better to remove the old one.
All DDF provided directly with deCONZ typically reside in
/usr/share/deCONZ/devices/on a Linux system and are loaded first. However, files residing in the home directory of the user running deCONZ (e.g./home/<DECONZUSER>/.local/share/dresden-elektronik/deCONZ/devices) will override the pre-packaged files to allow users to amend and keep their own files if desired.
For HA How to add/edit a DDF on Home assistant using text editor?