x server and application clients

Question

I need some clarifications about the X Server and the applications. As described in the wiki: X Window System protocols and architecture, "X server is go-between for the user and the client programs, ...,"

My question is, after a Linux PC starts, e.g., CentOS, at beginning, X Server starts to run (taking keyboard, mouse, monitors),

1. does X Server serve I/O for all the subsequent application programs? If not, how to make program A via X Server, while program B not via X Server?

2. what about other GUI programs, e.g. If I have a Qt program, does it also go via x server, like keyboard/monitor <-> X Server <-> Qt? If not, how is this situation handled?

Answer 1

1. Yes. Even the console applications will run in a terminal-like application that will be a client of X.

2. Yes. Qt programs do go through X.

Answer 2

X11 is a system to forward the GUI of a program, it works with every program of a system that supports X11. X11 pretty much forwards to a remote computer all the keys typed and all the clicks done on a computer, sends them to the remote computer and then the remote computer processes the subsequent actions (e.g. make a window appear). To sum up Xserver forwards the screen output of a program (buttons, text boxes etc.) and sends back to the remote machine all the inputs (e.g. clicks, keyboard keys pressed etc.)