I see your point, but I guarantee that the motherboard is more expensive than a beginner's book on digital electronics.

I don't remember which books I read when I was beginning, and I haven't had the need for electronics books for a while. Maybe someone else can recommend a book?

Also, you can find books and their reviews on many online bookstores such as Amazon, for example.

If you have a local library (I honestly don't know that much about Philippines to guess whether you do) you could try to go there and borrow a book.

No.

Please return to these forums when you have a solid understanding of the basics.

Asking a meaningful question here as opposed to asking one over email is potentially thousands of times more efficient.

Logic gates are preferably inside the IC for your device, so you don't explicitly have to deal with them. While the knowledge of gates is useful in the long run, you need a more practical (higher) level of understanding for your project.

The port expects one bit per each clock cycle. If your port baud rate is 9600 (9600 bits per second), the receiving device (your computer) reads one bit for 1/9600th of a second before moving to the next bit. When it has assembled a byte (generally 8 bits) and received the optional stop bit as well as error checking bit after the 8 data bits, it will tell the operating system that the port has data that can be read.

In order to assemble these bytes on the sending device using a specified clock rate, you need a logic gate array that does this. That logic gate array is most conveniently packaged in an IC chip so you don't have to make the gate array yourself using a lot of transistors. A programmable IC (PIC) gives you more flexibility in what your device can do by itself.

The same IC reads the inputs from your switches and combines them to the byte (8 bits) that gets sent to the wire, inserts the optional frame bits and handles the timing of individual bits by using an oscillating crystal or capacitors.

The voltages can be adjusted via simple resistors or potentiometers (variable resistors).

I don't know how to make this more clear, sorry.

Read the applied sensors documentation on this page. It will give you an idea of what you need to do.
If you need more info read Lamothe's video game programming book he covers all of this stuff in detail from i.e. how every voltage from an external controller chip communicates with a pc,etc.