NV is real time observing. There is no need for an external tablet, Laptop, or phone.
The most common devices are similar to the US PVS-14 and almost all device made today are very similar.
There is no external connection because the unit is fully self contained. The image if formed by a phosphor screen inside the device. The power is supplied by a battery mounted in the device. It is fully self contained.
The device has a built in eyepiece that focuses on the built in screen so you put your eye up to it and look though it.
The eye relief is quite generous.
The apparent field is 40 degrees.
The device has the same focal length as a 26mm-27mm eyepiece and magnification is 1x with the attached lens (no models sold in Europe have a field removable objective).
If you use the device afocally (which means holding it up or mounting it to a conventional eyepiece), since the magnification of the device itself is 1x, if you hold it up to or mount it to a conventional eyepiece, the view will have the power of the conventional eyepiece. For example, if you use a 55mm Plossl in your telescope and it gives 30x, then if you look into that eyepiece using your device, the view you see will be 30x. If you used a conventional eyepiece that gave a 70x view, then the view you would get using the device to look into that eyepiece, the view would be 70x. In both of these cases, regardless of the apparent field of the conventional eyepiece, the apparent field in the NV device would always still be 40 degrees, so your 55mm Plossl's 50 apparent degree field would be reduced to 40 apparent degrees.
If you want to capture an image, you can attach a cell phone to the back of the NVD using inexpensive cell phone holders and take an image of the view in the device eyepiece.
I usually just hand hold my device to an eyepiece for afocal viewing but if you are using Televue eyepieces or other eyepieces that have the ability to mount the Televue Dioptrix, then Televue sells an inexpensive adapter that will allow you to connect a PVS-14 directly to the eyepiece. There are also various other ways to mount to different eyepieces such as afocal camera platforms or custom adapters.