So I just need to use the commands.dat file?
Generally, no. The Commands.dat file is one of two configuration files for server settings, but it is not instrumental in setting up an Internet server instead of a LAN server. The only relevant part of the Commands.dat file would be if you were to use a different starting port than the default (the default is 27015), or if you have a multi-router setup in which case you may need to bind a specific IP (this does not apply to a vast majority of people).
Nothing has changed regarding the default ports that need to be forwarded (which are still 27015, 27016, and 27017). If you knew how to port forward three years ago, you should still be find. If not. refer to: PortForwarding.md.
Whether you launch a LAN server or an Internet server is determined when you launch your server. For an Internet server, your batch file will contain something akin to start "" "%~dp0ServerHelper.bat" +InternetServer/MyServer
. I am assuming that you're on a Windows OS, and as such you can refer to: ServerHosting.md#How_to_Launch_Server_on_Windows.
And can you explain what SteamCMD is and what it's used for?
SteamCMD is a CLI (command-line interface) version of Steam, developed by Steam. Many Steam games and server hosting providers use SteamCMD to run servers, as it doesn't include unnecessary Steam files and is more optimal for those running multiple servers at once, or for those who wish to run servers while not logged into their normal Steam account.
It is not necessary to use SteamCMD if you plan to just run one server at a time. In that case, you could just install the Unturned Dedicated Server app onto your Steam Library instead.