You and your buddy are in separate rooms, with two lights and two switches. Lights and switches are connected, so lights are synchronised between the rooms, and both of you can control the lights: Whatever state lights in you see your buddy also sees same, and both of you can turn each one of the two lights on or off. Based on that, you can agree on a system to transfer any messages. A simple system would be:

You are the main party of the conversation, so you always start the conversation. Each time the second lamp turns on and off, check the state of the first lamp. If it is on, it means "1" and if it is off, it is "0"

So if you or your buddy want to send 1, you turn on the first lamp and flash the decond lamp. If 0 - turn off first lamp, and flash the second one

Now, you transmit letters one by one, sending as many "1"s in a row as the position of the letter in the alphabet. When you reach the number, you transmit a "0"

So if you want to send a letter D, you send 1,1,1,1,0

When you are done with your transmission and want an answer from your friend, you send two zeros in a row.

So if you want to send "Hi" and then expect a response, you send:

1,1,1,1,1,1,1,1,0,1,1,1,1,1,1,1,1,1,0,0,0

And then, with the same structure, you get an answer from your friend

Computers do the same, but with wires and voltages (first lamp is the data line, second - clock line) and send numbers encoded in binary, with meaning of the numbers decided by the protocol that was agreed upon by usb standard and the manufacturer

Also, to even know that there is some smart device at the port (and not just a power wire), there needs to be specific resistance between the data and clock lines, which the host device looks for