Thanks for the input everyone, I understand the controls are very hard to "get", I'll try to rephrase:
As the player you are inside the mine, looking at a wall. There are 4 walls around you, and you can look around by pressing Q or E, or clicking the top buttons.
The view is also focused on a single floor, which you can see with the purple outline.
You can rotate the selected floor around you by pressing A or D, or clicking the left and right arrow buttons. Doing so will connect or disconnect pipes.
You can also select the upper or lower floor with W or S, or clicking the up and down arrow buttons.
Finally, you can **place mirrors on the junction tiles** by simply clicking on those tiles.
**If you don't place any mirror there is no way you can get to the 6th level.**
I hope this helps. We could have added more hints or animations in-game, but we're just programmers so it would have been tough.
And congrats @evogengames for managing to get to the last level, we know it is quite challenging! :D