Can you grant superuser privileges to an application? I was wondering wether it is possible since ES File Explorer has the root explorer option,  which I need to see where Termux got installed/from where it's running to be able to navigate (though it seems that I can't).    

So, is there a way to grant just it such privileges with adb,  or  at least a permission for viewing such files?
Your device needs to be rooted first 
You can only grant SU permissions when the application asks for it in a rooted environment. Forcing the app to ask for permissions is not trivial or likely to succeed . For details, see related question linked below
If you are not rooted, you can't bestow super user privileges to any app 


Related How to add root permission to some app by myself?