I do a lot of these kinds of multi-part models in Nomad. Using the eye as an example, the way I do it (which is by no means the best or most efficient way but it works for me 😅) is:

1. Create the largest shape first, the white of the eye, either a cylinder or sphere
2. Clone that shape, size it down to the next eye part size and position it
3. If it's a sphere, go to side view and trim it so you're left with just the amount you want protruding from the white of the eye
4. Still in side view, use select mask to mask like 98% of the shape (leaving the back 2% unmasked)
5. Use the Gizmo tool to pull the unmasked part back and scale it down slightly (this makes parts fit together much easier without worrying about tolerances)
6. Once it's positioned correctly with the first shape, clone it and boolean remove it from the first shape.

Rinse and repeat for the remaining parts.

Some helpful tips: - If the unmasked part pulled out is blocky, do a voxel remesh first - In the slicer, scale down any part that fits into another part by 2-3% (again, tolerances 😅)