Just try to keep the youtube modelers in perspective. Their job or part time job is making videos for youtube or building models on commission and reviewing products. Naturally they'll have a lot of stuff.
As for the paint jar opener...once you get a cap dried on and it rips the skin off your palm the first time you'll understand. Yes, they're overkill and a simple kitchen gripper can help you get a bite on the screw top...but for a couple dollars it's a decent price for a tool that can help with that.
Tools like the scribers can absolutely be replicated with a hobby knife...but a scriber tool is a "more appropriate" tool. It's built to make that specific task as easy as possible.
Many of us build up a paint collection as we need it. I have close to 130 colors in tamiya acrylics, testors enamels, and a few Vallejo colors. Some of them I just got because it was there and the paint jars are only ~$3-7 most of the time. Most of them were needed for a specific model. As silly as it sounds Grey, German Grey, and Japanese Grey are all subtly different and they're formulated to be as accurate to the real life counterparts as possible. Accuracy is a big part of historical military builders, some people go so far as to attempt to replicate specific subjects from the historical photos. Mixing primary colors of paint to a desired shade is hard to do at the small volume we'd use.
Starting out, the cheap way to go about getting paints is just buying the colors you need for specific models. Eventually you'll find you only need to buy one or two, or none, since you already have them. As far as tools, all you truly need starting out is:
hobby knife, sprue cutter (side cutter), a few files, a few tweezers, some sanding sticks in various grit, and a couple paint brushes
You can do most models with around 4 brushes. A fine pointed brush for details, a thicker pointed brush for larger details, a small flat brush, and a larger flat brush for painting larger areas smoothly. Eventually you might want a stiffer brush for doing the "dry brush" technique, or you might need some for decal solutions.
Under no circumstances do you need all the specialized tools that the youtubers have. It's just nice to have them if you get serious about the hobby and want a "better tool".