In the "Getting Started with Woodworking" video, the holes through the 4x4's were drilled from the back. That is, they start on the side opposite the precisely-positioned mark, and drill through to hit it. If they can do this, more power to them, but I can't drill through 3-1/2" of wood to emerge at a precise mark without a drill press - and not always then.
The shelf in the first picture is made of red oak plywood. You can choose the wood type, color and design as you like for your project. In case if you need more help understanding this project, you can refer the source link below. It discusses various items used, steps and tips and personal experience of the author who personally built a Zigzag shelf.
A board is considered “quarter-sawn” when the growth rings run, more or less, perpendicular to the face of the board. Quarter-sawn boards generally have straight grain and are less prone to shrinkage, compared to other boards. These factors don’t come into play with the 2x4s you use to frame a closet—but it does with the shelves and cabinetry you put into that closet; you want those boards to remain straight, flat and stable.
Luckily, we have also managed to find a detailed video tutorial of the Barn door project that illustrates the process of building a Barn door of your own. The steps and instructions in the video tutorial are different from the source links listed above. Actually, you can make different types of designs for your Barn door depending on which one you can afford easily and DIY on your own.
Clamping up four mitered corners is tricky. You can buy specialty clamps for this, but I make my own. Here’s how to do it. Start with a long 1×4, as it’s easier (and safer) to clamp for making the angled cuts than a short piece. Mark out the blocks, and then drill a 1-in. diameter hole in the center of each one. The hole prevents the blocks from getting glued to your project. Cut 45-degree angles tangent to the hole, and then cut the blocks free from the long board. We’ll walk you through how to make one for your home shop.