Umm,… it was two years ago so I don’t remember specifically how I did it but looking at it I can guess…
There would have been 3 main plugins to make the construction simple. All from Thomthom, Quad face tools, Vertex Tools and SUbD.
I would have constructed a basic rectangular framework of square tubes for the outer frame, then an array across for the seat and back, still in square tubes. All the tubes would need to checked and worked in such a way that they all fit together as Quads with no internal faces where they joined therefore creating one solid piece.
Once you have your ‘square’ solid then you start shaping it with Vertex Tools. Shrinking, stretching pulling etc to refine the shape all the while hitting SUbD every so often to see how the curved form looks, and find any errant extra edges, bad triangles or sections that just don’t work. QFT is pretty important in the part as it allows you to control the edges you want to select and add and remove loops and rings(horizontal and vertical edges) to help smooth and simplify the form.
Then when happy with the shape I would have deleted half of it, the half with any errors or malformed sections and mirrored it to make a symmetric chair.
Sometimes you can use a mirrored component to do the second half as you work but I have often find it is better to work on the whole shape and just snip and flip from time to time. This way you can get some nice feedback from the model that is more organic than a sterile mirror. Some little errors where you’ve grabbed the wrong vertices on the other side can show you a nicer version of what you had originally envisioned, or you can work the left and right differently and compare to find the better result.
All in all it’s probably taken me longer to write this than it would have taken to make it.