Primary battery vs USB vs 12v

What is the relationship between the main bike battery and the USB and 12v systems? The manual warns of not using 12v long as I think it comes off a smaller internal battery unrelated to the main bike engine battery. USB I’m not sure. And if one or both sockets come from the small bike battery, isn’t that recharged from the main one when the bike is on? Or only when in Ready state?

Basically, how might we tap into the fully charged main battery to run some decent power USB or 12v device for a longer period of time safely?

I have my radar detector plugged into the 12v all the time that I'm riding. I don't see any other option. So far, so good. Isn't the 12v meant to be plugged into by your heated riding gear? If so, how can it not be designed to be plugged into long term - as much as 80 miles can be deemed long term?