How do I remount USB Flash Drive or SD card? I noticed that I could remount my USB hard drive through terminal after I eject it by typing diskutil list to find the drive and diskutil mountDisk /dev/disk"X"...

So I tried to do the same with my SD card and my USB flash drive but...after I eject them, I cannot find them anymore through diskutil list.

I suspect this is because they are not "drive" until the system determines them as drives...which happens automatically when you plug them in - and the system remove those drives from the USB interfaces/SD card interfaces after I eject them.

So, the question is, how do I manually determine these USB interfaces/SD interfaces as "drives" so I can remount them. Thanks.
There's a difference between unmounting and ejecting.
I'm not familiar with the CLI of diskutil, so I'll explain it using Disk Utility.app

When you have a mounted SD card you can unmount it by clicking the Unmount button in the Toolbar:

The SD card stays in the list, but it is faded out.
Then if you want to remount it again, just select the SD card and click Mount: