The main file is startup-config.conf, which is read during bootup to configure the system. To assure that this file is saved, you need to initiate a shutdown from the web interface. If you don’t do that, there’s no guarantee that your configuration has been saved.
So do that first. Then you return to this page, and use the “Download” feature to download a copy of the file to your computer.
You can also keep multiple configuration files on the usg-50. There’s a copy feature, and a rename feature, so you can copy the startup-config.conf file to another file, alter you router, and then restore it to that earlier state using the “Apply” feature.
This could be useful if you need to reconfigure the network for special events.
Read the manual for complete information. The system is pretty sophisticated.