It's a big plywood box. Nothing fancy, right? Yes and no.
The exterior of the cabinet is going to be dressed up a bit, some of which is taking place now, some of which is taking place off site and will be addressed down the road. But the goal was to keep the outside of the cabinet free from any visible mechanical fasteners, or with as few as possible.
We started construction by getting a frame built on the floor, as that was the easiest point from which to begin and gave us the best opportunity to get a solid, square base from which we could build vertically. This didn't come without challenges however, as the floor slopes almost a full inch from one side of the cabinet to the other. As a result, the bottom face board of the cabinet had to have a line scribed relative to the floor and then run freehand through the table saw (nobody felt like dragging a band saw to the job site). Anybody that's ever done it will agree, trying to cut a wavy line with a monster of a saw designed mostly to cut ultra straight lines isn't super simple; it doesn't take much to get off line or have the cut get away from you. A circular saw might have been marginally easier, but with a circular saw you always want to cut on the bad side of the board (the side being cut is where the most tear-out will occur), and we had to scribe our line on the good side of the board which forced us into using the table saw for the cut.
With the bottom platform built, we ran a couple 2xs up the wall vertically. Again, the goal was minimal fasteners but with a wavy plaster wall, you're very much at the mercy of using however many anchors it takes to get the board solidly in place. We ended up using 4" concrete screws, which requires drilling pilot holes, but on more than 1 occasion, for whatever reason (something stupid solid in the masonry wall) we had to make multiple holes and screw-driving attempts to get the screws fully seated without blowing up a drill or impact driver.
With verticals in place, we could attach the side panels. Again, we had to scribe lines so the side panels would sit flush to the wavy plaster wall. Additionally, we had to make sure that not only did the side panel hug the wall, but the front edge of the side panel had to remain perfectly plumb. On top of that, because we had to go almost 11' feet up the wall, we had to make each side panel out of 2 separate pieces of plywood: an 8' piece, and a 3' piece. This all required a LOT of careful measuring and cutting.
Then it was on to the shelves. We had originally thought about simply dadoing the side panels to receive the shelves, but I didn't trust glue alone to hold that joint solid throughout construction, and shooting nails from the outside of the cabinet wasn't an option too many people were thrilled with, so we ended up just attaching cleats to the inside of the vertical pieces and back wall (which again required more hammer drilling and sometimes multiple attempts to find a spot in the wall that didn't hate our 4" Tapcons) and then nailing the shelves to those.
Oh, and each shelf board had to be notched to fit around 2 vertical 2xs, ductwork and a flu.
Long story short, this is nothing more than a big plywood box...that required just about every single board we used to build it to be custom cut.