Where to download the FP3-REL-Q-3.A.0136-.gms-7c69ec7e-user-fastbootimage.7z?

I have checked this article : Fairphone 3 unlocking without oem unlocking to unlock but there are two steps described as :

  • fastboot flash devinfo devinfo-unlocked.gpx
  • fastboot reboot bootloader
    and I have no idea what that means, are they commands I have to write somewhere ? But where ? I don’t get it. I am trying to find the meaning on internet, how to flash stuff on a phone but I can’t find anything clear. I have downloaded adb but I don’t know how to open it, there’s a sort of terminal window that shows up and then it’s gone in a matter of 1 second.

Can someone explain what I am missing ?