Neofetch will by default create a config file at $HOME/.config/neofetch/config.conf on first run. This file contains options to control all aspects of the output. The config file allows you to keep your customization between versions and allows you to easily share your customization with other people.
You can launch the script without a config file by using the flag --config none and you can specify a custom config location using --config path/to/config.conf.