The display is effectively the X server; there is exactly one display per X server. So multiple X servers can’t run simultaneously on the same display, and an X server can’t run simultaneously on multiple displays. (Strictly speaking, the latter point isn’t correct, but I don’t think there’s an X server which can serve multiple displays.)
https://www.x.org/archive/X11R6.8.0/doc/X.7.html#sect4 says a display can have multiple screens/monitors.
$DISPLAYspecifies a screen, not just a display, and is used in starting a X server or a X client. So does a X server start in a display or a screen? So does a X server start in a display or a screen?
https://unix.stackexchange.com/a/503884/674 has a diagram that
distinguishes screen and monitor, while https://www.x.org/archive/X11R6.8.0/doc/X.7.html#sect4 seems to say they are the same concept when explaining screen number. Which one is correct?
shows a X server covers all the screens in a display. So does a display server start in a display or a screen or a monitor?
An xserver can use a hardware framebuffer, a dummy framebuffer (Xvfb) or a window on another xserver (Xephyr). The latter two are examples of "virtual" xserver/display
Is a framebuffer associated with a display or a screen or a monitor?
Sorry I am still confused by the multiple concepts. Thanks.