I don't want to estimate storage space right now, but meshes can be stored very efficiently. For example, I think UE uses an optimization where vertex positions are heavily quantized to just a few bits within the meshlet's bounding box. Index buffers can be constructed to share the same vertices across LOD levels. Shading normals can be quantized quite a bit before shading artifacts become noticeable - if you even need them anymore at that triangle density.

If your triangles are at or below the size of a texel, texture values could even be looked up offline and stored in the vertex attributes directly rather than keeping the UV coordinates around, but that may not be a win.

Not necessarily. Nanite compresses meshes (including in-memory) _very_ heavily, and _also_ streams in only the visible mesh data.

In general, I wouldn't think of Nanite as "one thing". It's a combination of many, many different techniques that add up into some really good technology.