Here an example of some shrink wrapping - taking a complex mesh of composed of many parts and internal details (the engine and seats are all modelled) and wrapping it into a single closed solid mesh by taking the bounding box as the starting geometry and the disjoint mesh of many pieces as the target.
To not get people’s hopes up too much - this is pushing it to the limit - the shrink wrapping could still use some improvement for really complex models like this - it can still get a bit tangled when there are very fine details below the scale of the edge length. It’s also slower than simply remeshing an object in place - this one took almost a minute to process. I’m hoping this can be improved though.
This sort of thing can also be approached with isosurfacing, but setting the iso threshold high enough to unify the mesh and close any holes does tend to lose some of the detail. I think there’s also some interesting possibilities for combining the 2 approaches.
(car model from here)