How can I fill the inside of this helmet? I'm new to Blender and I have this helmet model that I want to fill the inside of, but I'm not sure how to do it or if it's even possible. I've tried searching for videos or guides but nothing seems to cover what I'm looking for.
As you can see it's quite a complex model with lots of curves and angles so not very straightforward (at least to me).
Also if possible I'd like to create a separate piece that follows the profile of the Visor and would be able to sit behind it.
I basically want to make this into a solid object.
So my answer works better if you've kept a low-poly version. Always keep a low-poly version somewhere, before applying the Subdivision Surface.
You can:

Select all the external vertices + the small inner extrusions:



Press Ctrli to invert the selection:



Delete:



Fill the gaps, with Grid Fill if possible:



For the bottom, extrude inwards and close the shape: