Some have a simple led level readout some have LCD.
I guess those with simple led level indicator will be easy to tap onto the pins and sense if they go active.
Those with LCD/Display I would imagine it would be simple enough to also monitor the pins to see what is displayed.
You could also perhaps use a current sense module, it would only tell you what is used and not what is left but if you wrote some clever calibration stuff it could be a fairly accurate method.
The latter is a very similar method to what is shown in the video you saw now that I watched it.
I can take some pictures of the various units I have and the PCB used on units with a percentage display and also some with simple led indicators for capacity. With luck may assist your desire to find a method to calculate remaining capacity.
There is also this eBay item that does offer a communication method of various stats via USB 292076965244. It is not a powerbank itself but could perhaps be used alongside of one.
Still waiting for mine to arrive but I would guess it will have some simple processor and again be much like what that fella in the video developed. Once they arrive I shall pop the hood and let you now what's inside if you want.