The first yellow panel isn’t the end of the demo but very close to it (there’s another place you can reach after rising Area 1 using the knowledge you’ve gained getting to the yellow panel and the fact Area 1 is now higher).
While the game up to the first yellow panel is mostly linear (you do a whole loop through the map, going through all 4 areas until you get to that panel), after that it is all non linear as you will have to explore the areas again as you use the yellow panels to reconfigure the connections between each area, creating new possible paths for you to reach the places that were unreachable in your first exploration of the map.
I presume that I am not supposed to reach the other Yellow Square panel.
The other yellow panel you’ve seen is indeed unreachable in the demo.