What is the relationship between framebuffer, VT, and tty?

I'm now studying what is the mechanism behind Ctrl+Alt+F1~F7.

I found some infomation talk about framebuffer, VT, and tty. I know framebuffer is an anstraction to graphic card manipulation and tty(and pts) is something a program interact with. I also find some people talking about VT(virtual terminal), but I don't know what is the relationship between them.

After searching on the web, I made a figure.

I think a VT is a set of monitor and keyboard. If I want to use many tty in one set(one VT), I have to switch between ttys because I only have one monitor to show them. This is VT Swich. But what is VT actually be? A driver, a module, or something else?

I guess if I have two set of monitors and keyboards connect to my computer, I can group them to form two VTs. And I can show my tty1 in monitor1 and control it use keyboard1, and tty2 with monitor2 and keyboard2. I can do VT Switch in both VT, too. Is that true?

And I think I can view framebuffer(fbcon + fbdev) as a graphic card driver, but it is design for console, not for general purpose(proprietary drivers).

Also, there are some special files under /dev

/dev/tty* and /dev/pts/* are for tty
echo can send input to the tty, and cat can recieve output from the tty.

/dev/vcs* are for screen of VT
cat can get the text in the screen, but use it under X get all zero(not empty).

/dev/fb* are for framebuffer
cat can get the whole pixel in the screen, but use it under X get all zero(not empty). I heard that X server doesn't use framebuffer, so what does it use?

Please tell me where I am wrong.