How the Web Client (process) reads data/commands from keyboard and write data to display?

Think the web browser just as the terminal.

Read command from keyboard:

Initially, a user opens the browser in one’s computer (just as how you open a terminal), then the browser engine starts running and executing its instructions, waiting for us to type a command like enter a URL in the address bar.

As we type the characters of a URL in the address bar, the web browser program reads each one into a register and then stores in memory. When we hit the enter on the keyboard, the web browser knows that we have finished typing the command. Then the web browser program will execute that command by running following instructions that will make TCP/IP connection with a HTTP request.

Below is the diagram I drew to help me put all the pieces together:

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Write data to the monitor:

After the web browser receives a HTTP response, it will read for example the HTML data into its memory, and then browser engine will use it to build the DOM object, rendering engine will pain the DOM and writing the output from memory to the display.