I'm a beginner in photography who just started using back button focusing.
There’s a flaw in your understanding. Back button focus is a method for engaging the autofocus motor. Decoupling it from the shutter release (which locks exposure in Av/Tv modes) allows you to very precisely control both the focus and exposure, especially in fast moving environments (sports/wildlife/street/etc).
Focus points tell the autofocus what’s most important to you.
Regardless of whether or not you use bbf, selecting focus points or areas can help the camera maintain focus on your intended target so that you can compose and get the shot you want.
Using bbf hasn’t changed the role focus points have in your shooting.
It is neither more nor less necessary to use focus point selection with back button focus than it is when activating focus with a half-press of the shutter button.
The only thing moving AF from a shutter button half press to a back button changes is the way you tell the camera when you want it to focus.
AF point selection is one way to tell the camera where you want it to focus. You can also use Zone/Area AF, or automatic AF selection which lets the camera focus on what it "thinks" most needs to be in focus.¹ These various AF selection methods are the same regardless of whether you initiate AF using a shutter half-press or a back button.
For more about back button AF, please see:
¹ Which rarely seems to be what the shooter wishes to be in focus.