I don't believe we have any established or concrete lore about those areas. Obviously there are train transport routes for materials and objects. There are also various heavy bulkheads that have nothing behind them on the map, but likely would allow the transport of larger materials throughout HCZ. Entrance zone likely has a few storage areas as well. In terms of maintenance areas, there are the entrance zone tunnels. But in HCZ there are also a few miscellaneous doors that probably have maintenance tunnels behind them as well. Ventilation is also very important, and probably sprawls throughout the map. For example, the giant fan on the roof of HCZ.
Feel free to get creative, cause we don't have anything official. The above is just my own conjecture.