However, as I have no knowledge in this matter, I can't understand how I'm going to configure it if there is no video output that allows me to configure these equipment’s, assuming that the mouse and keyboard connect to the USB port?
If I mount two 6TB hard drives, the storage capacity will appear as if it were a single 12TB drive, or will it only be possible to work on two 6TB drives separately?
Which connection port is advantageous for network sharing: Ethernet 1 Gbs or USB 3.2 gen 1 6Gbs?
A web interface is normally used to configure a NAS. You just connect it to the network, and then open the web interface by putting the IP address into your web browser. (How you find the IP may vary with different devices.)
A NAS such as the Fantec "CL" device will use some form of RAID to combine hard drive capacities and/or provide redundancy, and it's all seen as one location to your desktop, laptop and phone (a mapped drive letter). Normally if you use two drives, they are set up as a mirror (RAID1), with the content duplicated on both drives so that if one fails, there is still a copy on the other drive and you can just replace the bad drive. So in your example of two 6TB drives you would only have 6TB of capacity to use and it would appear as a single location to the devices that access it, unless you choose to configure it to look like multiple locations which doesn't seem like something you want. If you need 12TB, you would either get two 12TB drives, or need a NAS model that lets you use three 6TB drives in RAID5, which provides redundancy and capacity. It still will appear as only one location to your other devices. (RAID0 is another option to get 12TB with only two drives, which also increases performance, but it's not a good option because if one drive fails then all your data is lost.)
The "BIG" option on the device basically just combines the capacity of the two drives, without any redundancy or performance increases. It is still shown to your computers as a single location, and when you save a file it just gets saved to one drive or the other. If one of the drives fails, some of your files will be lost. (It could be most of them, it could be a few of them, it could be half, depending on how this model allocates them and how full it is.)
I don't see anything about the NAS only supporting SATA version 1. It doesn't specify the version at all, so it's likely either version 2 or 3. In any case, a SATA3 hard drive is backward compatible and will work, and you would probably be fine even with 12TB drives. Fantec's site's specs say "unlimited" so you might even be able to use the most recent sizes like 24GB.
https://fantecshop.de/en/p/fantec-cl-35b2
The front USB port on the NAS is used for connecting an external drive for backing up the files that are stored on the NAS, which is highly recommended. The port on the back if for connecting a printer. Neither of them is used for sharing the files to your computers.
A device like the Fantec "QB" unit is just used to connect multiple hard drives to your PC with a single USB cable. In this model, there is no RAID, so your computer would see two separate 6TB hard drives with two different drive letters. You would have to set up Windows file sharing on those drives in order to let the laptop and phone access the files. That means if your desktop gets turned off, the files are unavailable, and it would be two different shared folders. There are other models that perform RAID, so that your computer would only see it as a single USB drive with one drive letter, which would make it easier to share the files, but it still depends on the desktop computer for sharing.
As far as speed, the USB 3.2 Gen 1 connection would theoretically be faster, but since you'd only be accessing one drive at a time the actual speed would not be any better than 1Gb LAN. And if the desktop PC only has a 1Gb LAN connection, that would limit the file sharing speed even if the Fantec was faster. Even with RAID, unless you have at least 4 mechanical drives in the right kind of RAID configuration, 1Gb LAN is plenty of speed. So a NAS like the Fantec CL device is the recommendation between the two options you gave.
This is USB 2.0 old stuff from 2014 and not a real NAS. Dot not buy it. As suggested, acquire a Synology or QNAP NAS.
It's not an outdated device in technology, and it is a NAS. The USB 2.0 applies only to the port used for connecting an external drive for additional storage (probably rarely used) or for backups to an external drive, or for a printer. That says nothing about the NAS/network performance, only that it's a lower-end (meaning inexpensive) device where they didn't bother to put in USB3 for a port that doesn't really need to be fast. Plenty of modern systems like laptops still come with a USB 2.0 port because it costs less and there are still uses for it. Fantec still sells the device directly, and other than the USB port speed, the specs are pretty much the same as any other low-end 2-bay NAS on the market but at a very low price. (There's nothing about the CPU or RAM or OS in it, so it's not likely to be capable of running other applications and acting as a server, but that's clearly not the intent at this price point. It's JUST a storage server. )
The only thing that makes this device somewhat outdated is, as Fantec themselves acknowledge, the cloud services and storage that it connected to, which have been discontinued. If you didn't care about those in the first place, that's not an issue, nor is it something only Fantec has run into, and again, it's a CHEAP device.