Can objects be completely solid? Not like an eggshell, where it’s empty inside.
No……..
This sounds like a common misunderstanding about 3d modeling that a lot of people have early on. This is a thing where the intuitive mapping of real-world objects to 3d models breaks down. It’s not so much that the object is a shell, as that the surfaces kind of only have one side. So, if you remove part of a surface, the object seems “hollow.”
If you provide some more detail about how this is causing you trouble, someone can likely make suggestions on how to handle it.
For instance, one common issue is when you cut an object in two or delete part of it (e.g., with the slice tool). When you do that, you need to create a new surface to “close” the resulting pieces to have complete contiguous surfaces. In Nomad Sculpt, there are tool settings to make it try to do that automatically.