Well, I'm glad you think they're nice; thank you. And I do enjoy your articles and reviews, as well.
Now, when you want to emulate detailed mountains such as these, you're going to have to go to the source: nature itself. The appearance and structure of mountains, simply put, can be narrowed down to whether it's folded (like much of the appalachians), upwarped (i.e. the Matterhorn), fault block (like what's in Yosemite), or volcanic (mount fuji). What brush and approach we use and how we use it will often determine which kind of mountain it mirrors. The following technique is the latest one that I have been using lately. It emulates folded mountains the best. The next big factor is erosion. Water travelling down the sides of mountains form the smaller, downward sloping ridges that make the mountain have an appearance of a wrinkled napkin. On my survival island, the mountains only have a few scars from erosion, but typically they can be everywhere, as if the mountain itself is wrinkled. To do this, I used the normal raise/lower tool with a spike brush (circular) at 100% intensity. I would hold it on the desired area for a few seconds to create a sort of basic mountain, that is, nothing too different from a normal cone. Make sure it's tall enough to be called a mountain, of course. You can repeat this across a landscape to designate where you want various peaks across a range. Connect the peaks with the same brush (we use the same brush until the "craggy" step), raising it to create the ridge connecting the peaks that you would find appropriate. You can also leave the mountain freestanding, like a volcano. After that, I would decrease the brush size (same brush, same shape, same intensity) and move from the peak to the base, going outward until I would have a nice, transitioning ridge. Don't go back over it, just go straight from peak to base, or you'll get an awfully vertically wavy ridge. I would continue with this wherever I could until the mountain looked something like a starfish from a bird's-eye view. I would continue like this until I achiever the aformentioned "wrinkle" effect. After the desired shape is achieved, I would go over the mountain with a rough brush just to add a "craggy" effect, as if it is truly rocky. Now we take advantage of the "above" or "below" degree option, found under the "above" or "below" level number selector and the intensity slider. What this does is manipulate only the terrain that would form at a certain angle. For instance, a mountain could be about 45 degrees while flatlands would be 0 degrees. Think of slope and y/x, because the y and x are two different points on any terrain. For any snowy mountain, we need to keep in mind that the snow typically doesn't remain on steep slopes, only settling where it's flatter. We can recreate this here by, firstly, coating our mountain with stone where we want it, and secondly, checking "everything below 45 degrees". When you choose the frost or deep snow layer and begin painting, you'll discover that it will only apply to slopes that are a lot more gradual or flat. And since we already went over the mountain with a brush to make the "craggy" effect, the snow will often form in little pockets and artful strains that look close to identical to their real-world counterparts. That's just about everything.
Again, this is my latest technique, and I didn't really go all out on these mountains. All you need at this point really is the time and will to go into details that you would expect in the real world. Again, much of my approach is based on von Imbilio on youtube. He doesn't do any tutorials, but if you're careful and look closely, you can inherit is technique quickly, like I did. If you haven't heard of it already, you can look into World Machine for an alternate and more technical approach to MC terrain design.