They don't; no one really does. It makes things seem easier, but truly learning how to have taste and good design sense takes more initiative and effort than reading a guide.
And you can only do that by researching, observing, noticing patterns and methods, interacting with them, and concluding what does and does not work. It is also key to learn to trust your conclusions, because if not you'll always be searching for someone else to tell you what is "right" and what is "wrong" and that is how you perpetuate the lack of self-efficacy that is required to have good taste and design sense.
Finding a hub, stream, feed, etc of content that shows you both what you don't like and do like is crucial in my opinion to developing good taste. You need to know what works, what doesn't, and how your creative/development process can make use of that knowledge to create your own work. If you have no means to see examples that you can analyse, it will be immensely more difficult to learn.
Also, most design guides that make claims of objective knowledge and try to define trends/patterns are usually written by failures and are just attempts to gain attention and a feeling of superior knowledge. Don't read that self-mastubatory stuff.