His map seems to be made up from Resources from CC3+ Core, Perspectives 3, SS5 and SS6.

You can easily check where a symbol comes from by going to Symbols -> Symbol manager, selecting any symbol (including one of the red X's) and hitting the List button. This should show the file path to the art file used (among other information), and it is usually not too hard to figure out what it means, for example all SS6 resources have SS6 in the path.

You can do the same for fills by opening up the fill style dialog, go to the bitmap files tab, picking any fill in the list, and inspect the path shown in the appropriate field.

Just to confirm what others already said, yes, you can add additional details to local maps. That's the main idea, and the same thing you see in any printed atlas. The country maps only have the major details, but the regional and local maps have more details.

Obviously, the local maps shouldn't add features that are so large they should have been on the higher level map. For example, it would be weird to add a major river or a capital city if those weren't on the larger scale map, but adding smaller features, like settlements smaller/less important than those on the main map, smaller rivers, and so on is completely fine.

After all, if you don't add any detail to the local map, it hardly have a reason for existing at all.

As for naming, I think most features added to the map should be named. Names make it easier for those making lower-level maps to be consistent, and it makes a map into a map. After all, a map is for showing where things are, it is a map after all, not an aerial photo, and without names it gets kind of meaningless. Humans tend to name every feature around them, and my line of thinking is more along the line of "If it isn't worth naming, do it really belong on the map at all?". Some symbols may appear just for decoration and naturally don't get a name, but otherwise, most features should be named; settlements, rivers, oceans, forests, mountain/hill ranges, points of interest, major areas, etc.....

Detail maps will typically have more than enough new features to name.

When it comes to symbol sizes, I think an important part of a map is that information is legible when zoomed out. Exactly how large that is depends on the map obviously, but in most cases, it will mean the symbols will be covering more area than the settlement actually takes up.

Currently, the settlement symbols do look a bit small IMHO. They don't have to be scale 1, but I think they could probably be 0.5.

More "precision" is usually better done by making a new local map from parts of the main map, which can show more details, and "more correct" settlement scaling. Exactly where the cutoff point for detail level lies between those maps of course differ based on the map itself.

You can however make a drawing tool that emulates one of the basic tools. Just set it with the desired shape, and then set all properties of the tool to use "current properties". This will make it act just like a basic tool, but it will have the options like trace (provided the shape picked for the drawing tool supports it).

As have been pointed out above, this starts to happen because your entities have too many nodes. But I want to point out the reason why this happens in the first place, namely too much fractalization. When you draw fractalized entities, CC3+ places intermediate nodes to be able to make this fractal shape, and each level of fractalization doubles the amount of nodes. This quickly gets out of hand if your setting is too high. When you draw large landmasses, it is adviced to use a lower level of fracatlization. This can be reduced using SIMPLYFY as mentioned above, but it is probably best to draw with less fractalization in the first place.