I know some if not all followers have a default equipment setting that you cannot remove from them regularly. They all have infinite arrows and the housecarls and such have a default item set they revert to regardless of you having taken everything from their inventory (steel armor). But what I'm having trouble with is why the hell does the Ebony (Legendary) Bow I give them (housecarls and Aela) always get outmatched by a Hunting Bow that is magically inserted into their equipment whenever they want to range attack? Like the steel armor you cannot remove the Hunting Bow since it isn't in their trade menu to begin with, but when I use the console to showinventory, they have it equipped over the stronger bows. Is this Hunting Bow actually a special one that companions get that have better stats than stuff you forge/upgrade?
This removes their bow and however many arrows they had, and replaces it with a BOUND bow and a BOUND (and unlimited supply of) arrow. So far, Lydia has only reverted to her original bow & arrows once after doing this.
For ease of use, open notepad, paste those 4 lines of code into it, and save it in your skyrim/data folder as whatever filename you want. Then you can open the console, click on the NPC to target it, and type "bat filename" (replace filename with whatever you named the text file as) and it will execute all 4 of those commands automatically.