The Nikon banks system is not intuitive. It goes back to at least the D100 in Nikon's DSLR product line. I think they missed an opportunity to redesign the customization menus and make the system more user-friendly with the migration from SLR to mirrorless platforms. But that is water under the bridge, at this point.
I've been using banks since purchasing a D500 in May 2018. It was not a system that made much sense to me, initially. The lightbulb moment for me came a few weeks after receiving the camera when I realized you don't need to save or recall anything when using banks. Yes, there is a tool in the camera menu allowing settings (including your banks) to be saved or recalled. But neither needs to be done to use the banks.
Banks allow for customization of Nikon professional cameras. Four banks - A through D - are associated with two different areas of the camera menu. The first are the Shooting banks, which are accessible atop the Photo Shooting menu (camera icon). These banks store settings made in the Photo Shooting menu. A second collection of four banks are associated with the Custom Settings menu (pencil icon)j. These store settings made in that menu.
Both the Photo Shooting and Custom Settings menus have four banks: A, B, C, and D. They are not linked. However, it is common practice to use the banks in pairs. It's really a matter of which usage is the better match for how you shoot.
I use four bank pairs: (A) Wildlife, (B) Birds, (C) Landscape, and (D) Portrait. I've renamed the banks. Since I'm in full manual exposure mode all the time, my customizations are geared around the release mode; autofocus, AF area and subject detection modes; and custom assignments to the function buttons.
For me, the key to groking with Nikon banks system was wrapping my mind around the fact I don't have to save or recall anything. The camera "remembers" or stores the Shooting and Custom Settings you've chosen. Both are set to their respective A banks as a factory default. So, even if a person isn't intentionally using the banks system, they're still using and customizing banks.
Since a typical wildlife photo shoot starts at dawn for me, settings in my "Wildlife" Shooting bank are 1/50-second shutter speed, lens wide open, ISO 12800, and a WB setting of 7690K. These settings will at least put me in the ballpark of what's needed when there's just enough light to make a decent photo. By the end of the morning when the animal activity has subsided, I'll be at 1/1000, lens wide open, ISO 500-800, and a WB setting of 5260K. I'll reset to the settings for pre-dawn twilight before powering off the camera.