Your PC sound goes through PC speakers that has built-in amplifier.

This is using car amplifier + speaker to replace your PC speakers. The other one is using car head unit built-in amplifier + speakers to replace the PC speakers.

I don't see anything special here from my understanding

Why would one connect car stereo to PC. The article is about using car amp or car stereo as amp to amplify PC sound.

Car stereo has built-in amplifier already. Why need to connect to PC? I'm getting confused

If you want to play radio or song, you need to use the RCA output from the car stereo and you need an adapter to connect to the PC input because it has different type of connector. Remember that the RCA output from car stereo is Line Level signal. So, connect to the Line Level input of the PC

So, the picture is the back of your car stereo, I presumed. Looks like it has RCA connector (colored White and Red). This is the Line Out signal level. As you probably know:
White -- Left audio signal
Red -- Right audio signal

You just need to connect this signal to PC input (Line level input). You do not connect the wire that normally connect to the speaker because it is an amplified audio signal. It will fry your PC audio input.

So, just get the adapter wire (very common) to connect from the car stereo RCA Line Out signal to the PC Line In signal which is normally Blue in color. Refer to the link below:
http://manual.audacityteam.org/man/how_to_..._equipment.html

The power for the car stereo is, as you know, require 12V. This is available in PC because PC has 5V, 12V power rail. The main power to hardisk or CD ROM is 12V. tap it from there. You probably need PC Y power cable to split the 12V line from PC. Refer your car stereo manual for power connection.

Good luck.

I believe so. Power to the car stereo is just 12V and GND. You can easily tap the 12V and GND from PC.

The car stereo comes with its connector and on the other end just tinned exposed wire. You can easily buy the mating connector to plug to the PC power connector. It is more common than the car connector side.

You probably need crimping tool to make proper connector or just solder it (a bit soldering skill needed).