It basically allows you to set the tablet into it, and it allows you to connect more devices to the tablet (as the dock acts as a port-expansion system, so you can have it plugged into your TV, an external hard drive, keyboard, mouse, etc). Plus the dock should charge the tablet as well, while it's being used in the dock as well. Not sure if the dock is worth the price they ask for it. There are cheaper USB 3.0 alternatives out there that do the same thing for a fraction of the cost. But that's basically what the dock does--lets you plug more devices into it (you plug the devices into the docking station, and then slide the tablet into the docking station and the devices are then connected to the tablet while it is docked).

It has a built-in (soldered on) M.2 SATA drive, which is NOT upgradable. You will destroy the tablet trying to get it apart as much of it is actually glued together, and there are a ton of small screws. Not worth trying to upgrade even if you could because the chances of damaging the LCD is very good.

The 256GB advertised is the SSD (internal hard drive). It is like using a microSD card (which you can use as well in the tablet--there is a small slot under the fold-out stand in the back). You won't get the full 256GB of space on the internal SSD as some of it used by the operating system, so you may see about 220GB upon unboxing and starting it up for the first time. Still plenty of room for most people. But you can add a 256GB microSD card to the slot in the back to add storage. It won't be as fast, but should be fast enough for storing files, watching movies, etc.

Yes, download it from the Microsoft Store (I think it's free).