Basics of electronics / PCs

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HI,

Is there a good site / book / projects that explain what happens when
we send some information from the keyboard to a device that is
connected via a serial/usb port of the PC.

Say, for example, I hit the digit 6 for any device that is connected to
my PC via the serial port or USB port or any other port and the device
displays the digit 6 on the LCD or any other device that sets its time
counter to 6 seconds, or makes sound for 6 seconds, etc....

I know there is a micro controller or some kind of PCBs on the
electronic devices that understands any data that I send from the
computer, but I would like to know what exactly is happening and how
the devices understand the data that I send from the computer and
convert it into the appropriate commands to perform?

What software can I use to design the user interfaces to capture the
data in the computer and how does the device understand the same ?

Kindly let me know of any videos / books / online sites that explain in
detail of these simple and basic concepts.

Thanks in advance,
Sarita