Not sure what those black looking pieces are... Nice looking material though.
Not a stupid question in the least! It is a very good question.
The usual thing I have seen for making compartments are glass and acrylic pieces.
As for holding and separating substrates from each other while setting up, say a planted substrate in some areas and sand in the front, would be cardboard - left in place until ready to add more stones and plants.
Recently I used the industrial metal storage / pantry shelving plastic shelf liners which cover the bottom to make a flat surface for stacking your stuff. And I used one for a corner for planting substrate sections and a background on a tank, as well as a cover for the bottom glass (slight buffer against pointy rocks and scratching ones).
They come in various lengths and depths. Easy to cut. So far so good on them.
There are also thin flexible "cutting boards", actually like mats at the local dollar store $1.00 for 2 which is an easy fix for holding the substrate.
Interested to see what this material from the video might be.