Here's the settee in "Normal" use.
There are tracks fore and aft that support the slide out section. There are latches that lock it into place, you can see the holes that they fit into.
Here's a picture with all of the cushions removed. I did that for clarity, it's not necessary when setting it up. All of the cushions have long velcro strips on the back and stay in place.
Here the base has been slid out and the feet have dropped down. The base cushions are stiff enough that you don't have to fill the holes left by the legs. Poking hard I could barely feel them. The base does NOT slide out so far that it's not supported by the cabinet below it. Very strong.
And here we're set for sleeping. I left out one of the extra fill cushions again for clarity. Because the spot where the fill cushion goes is "taller" than the other base cushion, because the slide out section sits on top of the base, the cushions are also taller so you have a nice level surface. I love it when a builder pays attention to details!

Here's a final shot showing the filler cushion.
Any questions just let me know.