What to do when students bring me questions related to other courses and subjects?
There's no shame in saying you're not an expert in their domain specific knowledge.  It's more important to know your limitations than to give advice you're not directly qualified to give.  Direct them to a professor in that specific field.  

If you feel you have sufficient experience to give (at least) a starting point, by all means share it with them.  Just always make sure that you are clear about the limitations of your expertise.