In NodeBB, subcategories are referred to as child categories. In order to set up your subforum, you would create your necessary categories (one for mobile phones, and then others for android, ios, etc) and then set the parent category of your intended subforums to be your mobile phones category.
Hmm I don't think we expose isAdmin and isModerator globally to the templates so that you can them everywhere. Maybe we should, everything in the config object is exposed so if you open your console and type config any variable you see under that object you can use in the templates. You can also take a look at the routes by prepending api to the url like https://community.nodebb.org/api/topic/1917/users-shouldn-t-be-able-to-purge-their-posts/10 you can use stuff from there in the template as well.