It is a far cry from the ships in Star Citizen which come completely modelled. Scott Manley was showing some of the detail visible by clipping the camera through the hull of a t9 sized ship.
So far we did not even get unique cockpits for the ships expansion. If there are like you say plenty of people working on the internals they have kept it secret very well. The modularity btw should not be much
of an issue as the way Mr Braben described it there will be an elevator at the back of the cockpit to take you to the different rooms in the ship. Also what would there be to do? Combat does not take long
enough to allow a walk through the ship to put out fires, repairs maybe? there is a repair module. It is an exciting idea but not a simple one to implement.
https://www.youtube.com/watch?v=QPwZkd9iOeU
We don't know at what stage the ships in ED are at. We do know that they will be fully fleshed out when this is an important feature, like walking around you ship. Regarding what you can do, is the same as what you can do in Star Citizen or any other game where you can walk around.
Yes it is kept very secret, only those who are working on it knows what is being done, and of course DB himself.