Handling mime types.
Custom file type associations can be found in .config/mimeapps.list
.
mimeopen -d filename
Associate a file type to a default programm.
xdg-mime default nnn.desktop inode/directory
Set a different default file manager using a desktop file.f
To add a new mime type see the example for gemini.
To change the default user directories, edit ~/.config/user-dirs.dirs
.