If your intended system volume was going to require drivers that weren't built into WinNT, you needed to press F6 at a specific point during installation. This would allow you to load a driver that makes the volume visible / usable.
This process was specific to installing storage drivers needed for the system volume. All other driver installation happened elsewhere.
My memory says there was actually a "Press F6 to load system storage drivers" prompt or something displayed by the installer, but it wasn't displayed for all that long a time and I imagine it was effectively invisible for many people. I recall spamming F6 to make sure I wouldn't miss the prompt.
Actually there were two separate times during the installation process that you could press F6 to provide storage drivers. The first had no visible prompt! The second has the prompt you remember.
Here's how I remember it: The Windows CD itself had drivers built into the installer so that it could discover hardware. However, if you had a brand new storage controller, you might find that even Windows NT CD's installer wouldn't recognise it, so it would tell you that there were no storage devices found. To get around this you had to press F6 right at the start of the CD boot, before the Windows logo appeared. After a few seconds you could provide your storage drivers on a floppy disk, and the Windows installer program would continue to load. This time, the installer would recognise your disks. Then during the installation you would get a visible F6 prompt to provide your storage drivers. This allowed you to provide extra storage drivers that would be bundled with the installed OS.
Most people didn't know about the first F6, because I think NT installer had some sort of very basic, generic storage drivers that would work in most cases. If you had some very recent array controller, you would likely need to know about the "invisible F6 prompt".
This process was specific to installing storage drivers needed for the system volume. All other driver installation happened elsewhere.
My memory says there was actually a "Press F6 to load system storage drivers" prompt or something displayed by the installer, but it wasn't displayed for all that long a time and I imagine it was effectively invisible for many people. I recall spamming F6 to make sure I wouldn't miss the prompt.