I wanted to remap the buttons on my Onn box without having to unlock the bootloader or root and I was able to accomplish this by following a guide posted by Double_A_92 on XDA:
https://forum.xda-developers.com/t/guide-remapping-android-tv-remote-buttons.4433617/
It works well, though once in a while it will launch the original app, and you have to press the button twice to open the correct app. Also if you press the back button you will return to the original app instead of the previously running app. To avoid that just use the home button instead.
According to the guide, this method should work for other devices like the Xiaomi Mi Box.
I have tried other methods of remapping buttons, and they work, but they break the volume buttons. Instead of holding the volume button down, I have to press it several times to get to the volume I want. Does this method have that problem?