I shot the photos below while checking a colony that's living in one of my Bee Tree Hives in my high mountain valley. It was thrilling to see the bees doing so well.
While building their own comb from scratch - as opposed to simply drawing out the cells of man-made, pre-stamped foundation - honey bees will sometimes link together like this to span the spaces on which they're building the comb. This behavior is called "festooning" and is a perfect example of why Dr. Thomas Seeley (author of Honey Bee Democracy, and The Wisdom of the Hive) refers to a honey bee hive as "a treasure chest of mysteries." No one really knows why the bees are doing this. Any explanations, at this point, are probably still conjecture.
Above is a photo of the entire foundationless frame on which the bees are festooning and building comb. They will continue building down until they attach the comb at the bottom, and then also continue building out to the sides.
Notice that the cells they are constructing are not all the same size. This is just one of the advantages of providing foundationless frames to the bees as opposed to giving them foundation with the cell size pre-stamped on it - with foundationless frames the bees can build exactly the size cells they need on each frame, including, even, a mix of sizes like this.
You can see that they are building larger cells at the top and smaller cells at the bottom. The larger cells are where they will store honey in an arc at the top. Just below that, they will store an arc of pollen, and in the middle and at the bottom of this comb the queen will lay eggs and they will raise new bees there.
A foundationless frame allows the bees to construct the comb exactly the way they need it for the maximum health and efficiency of the colony.
Here's a close-up of the top section of the comb. You can see that they're already putting nectar in those larger cells. They will evaporate the nectar down from approximately 80% water to about 17% water. That will transform the nectar into honey and then they will cap the cells with wax to store it for future use. Simple evaporation is just part of the process of transforming nectar into honey. We know that the bees also add enzymes from their own bodies, and there may be other processes at work that we've yet to discover.